The social history of viruses describes the influence of viruses and viral infections on human history. Epidemics caused by viruses began when human behaviour changed during the Neolithic period, around 12,000 years ago, when humans developed more densely populated agricultural communities. This allowed viruses to spread rapidly and subsequently to become endemic. Viruses of plants and livestock also increased, and as humans became dependent on agriculture and farming, diseases such as potyviruses of potatoes and rinderpest of cattle had devastating consequences. Smallpox and measles viruses are among the oldest that infect humans. Having evolved from viruses that infected other animals, they first appeared in humans in Europe and North Africa thousands of years ago. The viruses were later carried to the New World by Europeans during the time of the Spanish Conquests, but the indigenous people had no natural resistance to the viruses and millions of them died during epidemics. Influenza pandemics have been recorded since 1580, and they have occurred with increasing frequency in subsequent centuries. The pandemic of 1918–19, in which 40–50 million died in less than a year, was one of the most devastating in history. Louis Pasteur and Edward Jenner were the first to develop vaccines to protect against viral infections. The nature of viruses remained unknown until the invention of the electron microscope in the 1930s, when the science of virology gained momentum. In the 20th century many diseases both old and new were found to be caused by viruses. There were epidemics of poliomyelitis that were only controlled following the development of a vaccine in the 1950s. HIV is one of the most pathogenic new viruses to have emerged in centuries. Although scientific interest in them arose because of the diseases they cause, most viruses are beneficial. Retroviruses drive evolution by transferring genes across species and bacteriophages play important roles in ecosystems and are essential to life.
In prehistory Over the past 50,000–100,000 years, as modern humans increased in numbers and dispersed throughout the world, new infectious diseases emerged, including those caused by viruses. Earlier, humans lived in small, isolated communities, and most epidemic diseases did not exist. Smallpox, which is the most lethal and devastating viral infection in history, first emerged among agricultural communities in India about 11,000 years ago. The virus, which only infected humans, probably descended from the poxviruses of rodents. Humans probably came into contact with these rodents, and some people became infected by the viruses they carried. When viruses cross this so-called "species barrier", their effects can be severe, and humans may have had little natural resistance. Contemporary humans lived in small communities, and those who succumbed to infection either died or developed immunity. This acquired immunity is only passed down to offspring temporarily, by antibodies in breast milk and other antibodies that cross the placenta from the mother's blood to the unborn child's. Therefore, sporadic outbreaks probably occurred in each generation. In about 9000 BC, when many people began to settle on the fertile flood plains of the River Nile, the population became dense enough for the virus to maintain a constant presence because of the high concentration of susceptible people. Other epidemics of viral diseases that depend on large concentrations of people, such as mumps, rubella and polio, also first occurred at this time. The Neolithic age, which began in the Middle East in about 9500 BC, was a time when humans became farmers. This agricultural revolution embraced the development of monoculture and presented an opportunity for the rapid spread of several species of plant viruses. The divergence and spread of sobemoviruses – southern bean mosaic virus – date from this time. The spread of the potyviruses of potatoes, and other fruits and vegetables, began about 6,600 years ago. About 10,000 years ago the humans who inhabited the lands around the Mediterranean basin began to domesticate wild animals. Pigs, cattle, goats, sheep, horses, camels, cats and dogs were all kept and bred in captivity. These animals would have brought their viruses with them. The transmission of viruses from animals to humans can occur, but such zoonotic infections are rare and subsequent human-to-human transmission of animal viruses is even rarer, although there are notable exceptions such as influenza. Most viruses are species-specific and would have posed no threat to humans. The rare epidemics of viral diseases originating in animals would have been short-lived because the viruses were not fully adapted to humans and the human populations were too small to maintain the chains of infection. Other, more ancient, viruses have been less of a threat. Herpes viruses first infected the ancestors of modern humans over 80 million years ago. Humans have developed a tolerance to these viruses, and most are infected with at least one species. Records of these milder virus infections are rare, but it is likely that early hominids suffered from colds, influenza and diarrhoea caused by viruses just as humans do today. More recently evolved viruses cause epidemics and pandemics – and it is these that history records.
In antiquity
Among the earliest records of a viral infection is an Egyptian stele thought to depict an Egyptian priest from the 18th Dynasty (1580–1350 BC) with a foot drop deformity characteristic of a poliovirus infection. The mummy of Siptah – a ruler during the 19th Dynasty – shows signs of poliomyelitis, and that of Ramesses V and some other Egyptian mummies buried over 3,000 years ago show evidence of smallpox. There was an epidemic of probable smallpox in Athens in 430 BC, in which a quarter of the Athenian army and many of the city's civilians died from the infection. The Antonine Plague of 165–180 CE, another probable smallpox pandemic, wiped out around five million people in the Roman Empire, which included Britain, Europe, the Middle East and North Africa. The pandemic began after Roman soldiers who were sent to suppress an uprising in what is now Iraq, plundered the city of Seleucia on the river Tigris and at the same time were infected. They brought the disease back to Rome and Europe where up to 5,000 people a day were fatally infected. At its height, the pandemic reached India and China. Measles is an old disease, but it was not until the 10th century that the Persian physician Muhammad ibn Zakariya al-Razi (865–925) – known as "Rhazes" – first identified it. Rhazes used the Arabic name hasbah (حصبة) for measles. It has had many other names including rubeola from the Latin word rubeus, "red", and morbilli, "small plague". The close similarities between measles virus, canine distemper virus and rinderpest virus have given rise to speculation that measles was first transmitted to humans from domesticated dogs or cattle. The measles virus appears to have fully diverged from the then-widespread rinderpest virus by the 12th century. A measles infection confers lifelong immunity. Therefore, the virus requires a high population density to become endemic, and this probably did not occur in the Neolithic age. Following the emergence of the virus in the Middle East, it reached India by 2500 BC. Measles was so common in children at the time that it was not recognised as a disease. In Egyptian hieroglyphs it was described as a normal stage of human development. One of the earliest descriptions of a virus-infected plant can be found in a poem written by the Japanese Empress Kōken (718–770), in which she describes a plant in summer with yellowing leaves. The plant, later identified as Eupatorium lindleyanum, is often infected with tomato yellow leaf curl virus.
Middle Ages
The rapidly growing population of Europe and the rising concentrations of people in its towns and cities became a fertile ground for many infectious and contagious diseases, of which the Black Death – a bacterial infection – is probably the most notorious. Except for smallpox and influenza, documented outbreaks of infections now known to be caused by viruses were rare. Rabies, a disease that had been recognised for over 4,000 years, was rife in Europe, and continued to be so until the development of a vaccine by Louis Pasteur in 1886. The average life expectancy in Europe during the Middle Ages was 35 years; 60% of children died before the age of 16, many of them during their first 6 years of life. Physicians – what few there were – relied as much on astrology as they did on their limited medical knowledge. Some treatments for infections consisted of ointments prepared from cats that had been roasted in hedgehog fat. Among the plethora of diseases that caused childhood death were measles, influenza and smallpox. The Crusades and the Muslim conquests aided the spread of smallpox, which was the cause of frequent epidemics in Europe following its introduction to the continent between the fifth and seventh centuries. Measles was endemic throughout the highly populated countries of Europe, North Africa and the Middle East. In England the disease, then called "mezils", was first described in the 13th century, and it was probably one of the 49 plagues that occurred between 526 and 1087. Rinderpest, which is caused by a virus closely related to measles virus, is a disease of cattle known since Roman times. The disease, which originated in Asia, was first brought to Europe by the invading Huns in 370. Later invasions of Mongols, led by Genghis Khan and his army, started pandemics in Europe in 1222, 1233 and 1238. The infection subsequently reached England following the importation of cattle from the continent. At the time rinderpest was a devastating disease with a mortality rate of 80–90%. The resulting loss of cattle caused famine.
Early to late modern period A short time after Henry Tudor's victory at the Battle of Bosworth on 22 August 1485, his army suddenly went down with "the English sweat", which contemporary observers described as a new disease. The disease, which was unusual in that it mainly affected the affluent, might have originated in France where Henry VII had recruited soldiers for his army. An epidemic hit London in the hot summer of 1508. Affected people died within a day, and there were deaths throughout the city. The streets were deserted apart from carts transporting bodies, and King Henry declared the city off limits except for physicians and apothecaries. The disease spread to Europe, arriving in Hamburg in July 1529 where one to two thousand people died within the first few weeks. During the following months it wreaked havoc in Prussia, Switzerland, and northern Europe. The last outbreak was in England in 1556. The disease – which killed tens of thousands of people – was probably influenza or a similar viral infection, but records from the time when medicine was not a science can be unreliable. As medicine became a science, the descriptions of disease became less vague. Although medicine could do little at the time to alleviate the suffering of those infected, measures to control the spread of diseases were used. Restrictions on trade and travel were implemented, stricken families were isolated from their communities, buildings were fumigated and livestock killed. References to influenza infections date from the late 15th and early 16th centuries, but infections almost certainly occurred long before then. In 1173, an epidemic occurred that was possibly the first in Europe, and in 1493, an outbreak of what is now thought to be swine influenza, struck Native Americans in Hispaniola. There is some evidence to suggest that source of the infection was pigs on Columbus's ships. During an influenza epidemic that occurred in England between 1557 and 1559, five per cent of the population – about 150,000 – died from the infection. The mortality rate was nearly five times that of the 1918–19 pandemic. The first pandemic that was reliably recorded began in July 1580 and swept across Europe, Africa, and Asia. The mortality rate was high – 8,000 died in Rome. The next three pandemics occurred in the 18th century, including that during 1781–82, which was probably the most devastating in history. This began in November 1781 in China and reached Moscow in December. In February 1782 it hit Saint Petersburg, and by May it had reached Denmark. Within six weeks, 75 per cent of the British population were infected and the pandemic soon spread to the Americas.
The Americas and Australia remained free of measles and smallpox until the arrival of European colonists between the 15th and 18th centuries. Along with measles and influenza, smallpox was taken to the Americas by the Spanish. Smallpox was endemic in Spain, having been introduced by the Moors from Africa. In 1519, an epidemic of smallpox broke out in the Aztec capital Tenochtitlan in Mexico. This was started by the army of Pánfilo de Narváez, who followed Hernán Cortés from Cuba and had an African slave with smallpox aboard his ship. When the Spanish finally entered the capital in the summer of 1521, they saw it strewn with the bodies of smallpox victims. The epidemic, and those that followed during 1545–1548 and 1576–1581, eventually killed more than half of the native population. Most of the Spanish were immune; with his army of fewer than 900 men it would not have been possible for Cortés to defeat the Aztecs and conquer Mexico without the help of smallpox. Many Native American populations were devastated later by the inadvertent spread of diseases introduced by Europeans. In the 150 years that followed Columbus's arrival in 1492, the Native American population of North America was reduced by 80 per cent from diseases, including measles, smallpox and influenza. The damage done by these viruses significantly aided European attempts to displace and conquer the native population. By the 18th century, smallpox was endemic in Europe. There were five epidemics in London between 1719 and 1746, and large outbreaks occurred in other major European cities. By the end of the century about 400,000 Europeans were dying from the disease each year. It reached South Africa in 1713, having been carried by ships from India, and in 1789 the disease struck Australia. In the 19th century, smallpox became the single most important cause of death of the Australian Aborigines. In 1546 Girolamo Fracastoro (1478–1553) wrote a classic description of measles. He thought the disease was caused by "seeds" (seminaria) that were spread from person to person. An epidemic hit London in 1670, recorded by Thomas Sydenham (1624–1689), who thought it was caused by toxic vapours emanating from the earth. His theory was wrong but he was a skilled observer and kept meticulous records. Yellow fever is an often lethal disease caused by a flavivirus. The virus is transmitted to humans by mosquitoes (Aedes aegypti) and first appeared over 3,000 years ago. In 1647, the first recorded epidemic occurred on Barbados and was called "Barbados distemper" by John Winthrop, who was the governor of the island at the time. He passed quarantine laws to protect the people – the first ever such laws in North America. Further epidemics of the disease occurred in North America in the 17th, 18th and 19th centuries. The first known cases of dengue fever occurred in Indonesia and Egypt in 1779. Trade ships brought the disease to the US, where an epidemic occurred in Philadelphia in 1780. Newly emerging infectious diseases (EIDs) have been posing an increasingly significant threat to human health. The majority of EIDs are of zoonotic origin, for which human population increase and the intensification of animal farming as well as of wild animal environments are causative in part.
Many paintings can be found in the museums of Europe depicting tulips with attractive coloured stripes. Most, such as the still life studies of Johannes Bosschaert, were painted during the 17th century. These flowers were particularly popular and became sought after by those who could afford them. At the peak of this tulip mania in the 1630s, one bulb could cost as much as a house. It was not known at the time that the stripes were caused by a plant virus, which became known as the tulip breaking virus, accidentally transferred by humans to tulips from jasmine. Weakened by the virus, the plants turned out to be a poor investment. Only a few bulbs produced flowers with the attractive characteristics of their parent plants. Until the Irish Great Famine of 1845–1852, the commonest cause of disease in potatoes was not the mould that causes blight, it was a virus. The disease, called "curl", is caused by potato leafroll virus, and it was widespread in England in the 1770s, where it destroyed 75 per cent of the potato crop. At that time, the Irish potato crop remained relatively unscathed.
Discovery of vaccination
Smallpox Lady Mary Wortley Montagu (1689–1762) was an aristocrat, a writer and the wife of a Member of Parliament. In 1716, her husband, Edward Wortley Montagu, was appointed British Ambassador in Istanbul. She followed him there and two weeks after her arrival discovered the local practice of protection against smallpox by variolation – the injection of pus from people with smallpox into the skin. Her younger brother had died of smallpox, and she too had had the disease. Determined to spare her five-year-old son Edward from similar suffering, she ordered the embassy surgeon Charles Maitland to variolate him. On her return to London, she asked Maitland to variolate her four-year-old daughter in the presence of the king's physicians. Later, Montagu persuaded the Prince and Princess of Wales to sponsor a public demonstration of the procedure. Six prisoners who had been condemned to death and were awaiting execution at Newgate Prison were offered a full pardon for serving as the subjects of the public experiment. They accepted and were variolated in 1721. All the prisoners recovered from the procedure. To test its protective effect one of them, a nineteen-year-old woman, was ordered to sleep in the same bed as a ten-year-old with smallpox for six weeks. She did not contract the disease. The experiment was repeated on eleven orphan children, all of whom survived the ordeal, and by 1722 even King George I's grandchildren had been inoculated. The practice was not entirely safe and there was a one in fifty chance of death. The procedure was expensive; some medical practitioners charged between £5 and £10 and some sold the method to other practitioners for fees between £50 and £100, or for half of the profits. Variolation became a lucrative franchise, but it remained beyond the means of many until the late 1770s. At the time nothing was known about viruses or the immune system, and no one knew how the procedure afforded protection.
Edward Jenner (1749–1823), a British rural physician, was variolated as a boy. He had suffered greatly from the ordeal but survived fully protected from smallpox. Jenner knew of a local belief that dairy workers who had contracted a relatively mild infection called cowpox were immune to smallpox. He decided to test the theory (although he was probably not the first to do so). On 14 May 1796 he selected "a healthy boy, about eight years old for the purpose of inoculation for the Cow Pox". The boy, James Phipps (1788–1853), survived the experimental inoculation with cowpox virus and developed only a mild fever. On 1 July 1796, Jenner took some "smallpox matter" (probably infected pus) and repeatedly inoculated Phipps's arms with it. Phipps survived and was subsequently inoculated with smallpox more than 20 times without succumbing to the disease. Vaccination – the word is derived from the Latin vacca meaning "cow" – had been invented. Jenner's method was soon shown to be safer than variolation, and by 1801 more than 100,000 people had been vaccinated. Despite objections from those medical practitioners who still practised variolation, and who foresaw a decline in their income, free vaccination of the poor was introduced in the UK in 1840. Because of associated deaths, variolation was declared illegal in the same year. Vaccination was made compulsory in England and Wales by the Vaccination Act 1853 (16 & 17 Vict. c. 100), and parents could be fined £1 if their children were not vaccinated before they were three months of age. The law was not adequately enforced, and the system for providing vaccinations, unchanged since 1840, was ineffective. After an early compliance by the population only a small proportion were vaccinated. Compulsory vaccination was not well received and, following protests, the Anti-Vaccination League and the Anti-Compulsory Vaccination League were formed in 1866. Following the anti-vaccination campaigns there was a severe outbreak of smallpox in Gloucester in 1895, the city's first in twenty years; 434 people died, including 281 children. Despite this, the British government conceded to the protesters and the Vaccination Act 1898 (61 & 62 Vict. c. 49) abolished fines and made provision for a "conscientious objector" clause – the first use of the term – for parents who did not believe in vaccination. During the following year, 250,000 objections were granted, and by 1912 less than half of the population of newborns were being vaccinated. By 1948, smallpox vaccination was no longer compulsory in the UK.
Rabies
Rabies is an often fatal disease caused by the infection of mammals with rabies virus. In the 21st century it is mainly a disease that affects wild mammals such as foxes and bats, but it is one of the oldest known virus diseases: rabies is a Sanskrit word (rabhas) that dates from 3000 BC, which means "madness" or "rage", and the disease has been known for over 4000 years. Descriptions of rabies can be found in Mesopotamian texts, and the ancient Greeks called it "lyssa" or "lytta", meaning "madness". References to rabies can be found in the Laws of Eshnunna, which date from 2300 BC. Aristotle (384–322 BC) wrote one of the earliest undisputed descriptions of the disease and how it was passed to humans. Celsus, in the first century AD, first recorded the symptom called hydrophobia and suggested that the saliva of infected animals and humans contained a slime or poison – to describe this he invented the word "virus". Rabies does not cause epidemics, but the infection was greatly feared because of its terrible symptoms, which include insanity, hydrophobia and death. In France during the time of Louis Pasteur (1822–1895) there were only a few hundred rabies infections in humans each year, but cures were desperately sought. Aware of the possible danger, Pasteur began to look for the "microbe" in mad dogs. Pasteur showed that when the dried spinal cords from dogs that had died from rabies were crushed and injected into healthy dogs they did not become infected. He repeated the experiment several times on the same dog with tissue that had been dried for fewer and fewer days, until the dog survived even after injections of fresh rabies-infected spinal tissue. Pasteur had immunised the dog against rabies, as he later did with 50 more.
Although Pasteur had little idea how his method worked, he tested it on a boy, Joseph Meister (1876–1940), who was brought to Pasteur by his mother on 6 July 1885. He was covered in bites, having been set upon by a mad dog. Meister's mother begged Pasteur to help her son. Pasteur was a scientist, not a physician, and he was well aware of the consequences for him if things were to go wrong. He nevertheless decided to help the boy and injected him with increasingly virulent rabid rabbit spinal tissue over the following 10 days. Later Pasteur wrote, "as the death of this child appeared inevitable, I decided, not without deep and severe unease ... to try out on Joseph Meister the procedure, which had consistently worked on dogs". Meister recovered and returned home with his mother on 27 July. Pasteur successfully treated a second boy in October that same year; Jean-Baptiste Jupille (1869–1923) was a 15-year-old shepherd boy who had been severely bitten as he tried to protect other children from a rabid dog. Pasteur's method of treatment remained in use for over 50 years. Little was known about the cause of the disease until 1903 when Adelchi Negri (1876–1912) first saw microscopic lesions – now called Negri bodies – in the brains of rabid animals. He wrongly thought they were protozoan parasites. Paul Remlinger (1871–1964) soon showed by filtration experiments that they were much smaller than protozoa, and even smaller than bacteria. Thirty years later, Negri bodies were shown to be accumulations of particles 100–150 nanometres long, now known to be the size of rhabdovirus particles – the virus that causes rabies.
20th and 21st centuries
At the turn of the 20th century, evidence for the existence of viruses was obtained from experiments with filters that had pores too small for bacteria to pass through; the term "filterable virus" was coined to describe them. Until the 1930s most scientists believed that viruses were small bacteria, but following the invention of the electron microscope in 1931 they were shown to be completely different, to a degree that not all scientists were convinced they were anything other than accumulations of toxic proteins. The situation changed radically when it was discovered that viruses contain genetic material in the form of DNA or RNA. Once they were recognised as distinct biological entities they were soon shown to be the cause of numerous infections of plants, animals and even bacteria. Of the many diseases of humans that were found to be caused by viruses in the 20th century one, smallpox, has been eradicated. The diseases caused by viruses such as HIV and influenza virus have proved to be more difficult to control. Other diseases, such as those caused by arboviruses, are presenting new challenges. As humans have changed their behaviour during history, so have viruses. In ancient times the human population was too small for pandemics to occur and, in the case of some viruses, too small for them to survive. In the 20th and 21st century increasing population densities, revolutionary changes in agriculture and farming methods, and high speed travel have contributed to the spread of new viruses and the re-appearance of old ones. Like smallpox, some viral diseases might be conquered, but new ones, such as severe acute respiratory syndrome (SARS), will continue to emerge. Although vaccines are still the most powerful weapon against viruses, in recent decades antiviral drugs have been developed to specifically target viruses as they replicate in their hosts. The 2009 influenza pandemic showed how rapidly new strains of viruses continue to spread around the world, despite efforts to contain them. Advances in virus discovery and control continue to be made. Human metapneumovirus, which is a cause of respiratory infections including pneumonia, was discovered in 2001. A vaccine for the papillomaviruses that cause cervical cancer was developed between 2002 and 2006. In 2005, human T lymphotropic viruses 3 and 4 were discovered. In 2008 the WHO Global Polio Eradication Initiative was re-launched with a plan to eradicate poliomyelitis by 2015. In 2010, the largest virus, Megavirus chilensis was discovered to infect amoebae. These giant viruses have renewed interest in the role viruses play in evolution.
Smallpox eradication
Smallpox virus was a major cause of death in the 20th century, killing about 300 million people. It has probably killed more humans than any other virus. In 1966 an agreement was reached by the World Health Assembly (the decision-making body of the World Health Organization) to start an "intensified smallpox eradication programme" and attempt to eradicate the disease within ten years. At the time, smallpox was still endemic in 31 countries including Brazil, the whole of the Indian sub-continent, Indonesia and sub-Saharan Africa. This ambitious goal was considered achievable for several reasons: the vaccine afforded exceptional protection; there was only one type of the virus; there were no animals that naturally carried it; the incubation period of the infection was known and rarely varied from 12 days; and infections always gave rise to symptoms, so it was clear who had the disease. Following mass vaccinations, disease detection and containment were central to the eradication campaign. As soon as cases were detected, they were isolated as were their close contacts, who were vaccinated. Successes came quickly; by 1970 smallpox was no longer endemic in western Africa, nor, by 1971, in Brazil. By 1973, smallpox remained endemic only in the Indian sub-continent, Botswana and Ethiopia. Finally, after 13 years of coordinated disease surveillance and vaccination campaigns throughout the world, the World Health Organization declared smallpox eradicated in 1979. Although the main weapon used was vaccinia virus, which was used as the vaccine, no one seems to know exactly where vaccinia virus came from; it is not the strain of cowpox that Edward Jenner used, and it is not a weakened form of smallpox. The eradication campaign led to the death of Janet Parker (c. 1938–1978) and the subsequent suicide of the smallpox expert Henry Bedson (1930–1978). Parker was an employee of the University of Birmingham who worked in the same building as Bedson's smallpox laboratory. She was infected by a strain of smallpox virus that Bedson's team had been investigating. Ashamed of the accident and having blamed himself for it, Bedson committed suicide. Before the September 11 attacks on the United States in 2001, the World Health Organization proposed the destruction of all the known remaining stocks of smallpox virus that were kept in laboratories in the US and Russia. Fears of bioterrorism using smallpox virus and the possible need for the virus in the development of drugs to treat the infection have put an end to this plan. Had the destruction gone ahead, smallpox virus might have been the first to be made extinct by human intervention.
Measles
Measles was a rare – although most often fatal – infection in South Africa in the early nineteenth century but epidemics increased in frequency from the 1850s. During the Second Boer War (1899−1902) measles was rife among the prisoners in the British concentration camps and accounted for thousands of deaths. This fatality rate in the camps was ten times greater than among British casualties. Before the introduction of vaccination in the US in the 1960s there were more than 500,000 cases each year resulting in about 400 deaths. In developed countries children were mainly infected between the ages of three and five years old, but in developing countries half the children were infected before the age of two. In the US and the UK, there were regular annual or biannual epidemics of the disease, which depended on the number of children born each year. The current epidemic strain evolved in the first part of the 20th century – probably between 1908 and 1943.
In London between 1950 and 1968 there were epidemics every two years, but in Liverpool, which had a higher birth rate, there was an annual cycle of epidemics. During the Great Depression in the US before the Second World War the birth rate was low, and epidemics of measles were sporadic. After the war the birth rate increased, and epidemics occurred regularly every two years. In developing countries with very high birth rates, epidemics occurred every year. Measles is still a major problem in densely populated, less-developed countries with high birth rates and lacking effective vaccination campaigns. By the mid-1970s, following a mass vaccination programme that was known as "make measles a memory", the incidence of measles in the US had fallen by 90 per cent. Similar vaccination campaigns in other countries have reduced the levels of infection by 99 per cent over the past 50 years. Susceptible individuals remain a source of infection and include those who have migrated from countries with ineffective vaccination schedules, or who refuse the vaccine or choose not to have their children vaccinated. Humans are the only natural host of measles virus. Immunity to the disease following an infection is lifelong; that afforded by vaccination is long term but eventually wanes. The use of the vaccine has been controversial. In 1998, Andrew Wakefield and his colleagues published a fraudulent research paper and he claimed to link the MMR vaccine with autism. The study was widely reported and fed concern about the safety of vaccinations. Wakefield's research was identified as fraudulent and in 2010, he was struck off the UK medical register and can no longer practise medicine in the UK. In the wake of the controversy, the MMR vaccination rate in the UK fell from 92 per cent in 1995, to less than 80 per cent in 2003. Cases of measles rose from 56 in 1998 to 1370 in 2008, and similar increases occurred throughout Europe. In April 2013, an epidemic of measles in Wales in the UK broke out, which mainly affected teenagers who had not been vaccinated. Despite this controversy, measles has been eliminated from Finland, Sweden and Cuba. Japan abolished mandatory vaccination in 1992, and in 1995–1997 more than 200,000 cases were reported in the country. Measles remains a public health problem in Japan, where it is now endemic; a National Measles Elimination Plan was established in December 2007, with a view to eliminating the disease from the country. The possibility of global elimination of measles has been debated in medical literature since the introduction of the vaccine in the 1960s. Should the current campaign to eradicate poliomyelitis be successful, it is likely that the debate will be renewed.
Poliomyelitis
During the summers of the mid-20th century, parents in the US and Europe dreaded the annual appearance of poliomyelitis (or polio), which was commonly known as "infantile paralysis". The disease was rare at the beginning of the century, and worldwide there were only a few thousand cases per year, but by the 1950s there were 60,000 cases each year in the US alone and an average of 2,300 in England and Wales. During 1916 and 1917 there had been a major epidemic in the US; 27,000 cases and 6,000 deaths were recorded, with 9,000 cases in New York City. At the time nobody knew how the virus was spreading. Many of the city's inhabitants, including scientists, thought that impoverished slum-dwelling immigrants were to blame even though the prevalence of the disease was higher in the more prosperous districts such as Staten Island – a pattern that had also been seen in cities like Philadelphia. Many other industrialised countries were affected at the same time. In particular, before the outbreaks in the US, large epidemics had occurred in Sweden. The reason for the rise of polio in industrialised countries in the 20th century has never been fully explained. The disease is caused by a virus that is passed from person to person by the faecal-oral route, and naturally infects only humans. It is a paradox that it became a problem during times of improved sanitation and increasing affluence. Although the virus was discovered at the beginning of the 20th century, its ubiquity was unrecognised until the 1950s. It is now known that fewer than two per cent of individuals who are infected develop the disease, and most infections are mild. During epidemics the virus was effectively everywhere, which explains why public health officials were unable to isolate a source. Following the development of vaccines in the mid-1950s, mass vaccination campaigns took place in many countries. In the US, after a campaign promoted by the March of Dimes, the annual number of polio cases fell dramatically; the last outbreak was in 1979. In 1988 the World Health Organization along with others launched the Global Polio Eradication Initiative, and by 1994 the Americas were declared to be free of disease, followed by the Pacific region in 2000 and Europe in 2003. At the end of 2012, only 223 cases were reported by the World Health Organization. Mainly poliovirus type 1 infections, 122 occurred in Nigeria, one in Chad, 58 in Pakistan and 37 in Afghanistan. Vaccination teams often face danger; seven vaccinators were murdered in Pakistan and nine in Nigeria at the beginning of 2013. In Pakistan, the campaign was further hampered by the murder on 26 February 2013 of a police officer who was providing security.
AIDS
The human immunodeficiency virus (HIV) is the virus that – when the infection is not treated – can cause AIDS (acquired immunodeficiency syndrome). Most virologists believe that HIV originated in Kinshasa in the Democratic Republic of Congo during the 20th century, and over 70 million individuals have been infected by the virus. By 2011, an estimated 35 million had died from AIDS, making it one of the most destructive epidemics in recorded history. HIV-1 is one of the most significant viruses to have emerged in the last quarter of the 20th century. When, in 1981, a scientific article was published that reported the deaths of five young gay men, no one knew that they had died from AIDS. The full scale of the epidemic – and that the virus had been silently emerging over several decades – was not known. HIV crossed the species barrier between chimpanzees and humans in Africa in the early decades of the 20th century. During the years that followed there were enormous social changes and turmoil in Africa. Population shifts were unprecedented as vast numbers of people moved from rural farms to the expanding cities, and the virus was spread from remote regions to densely populated urban conurbations. The incubation period for AIDS is around 10 years, so a global epidemic starting in the early 1980s is credible. At this time there was much scapegoating and stigmatisation. The "out of Africa" theory for the origin of the HIV pandemic was not well received by Africans, who felt that the "blame" was misplaced. This led the World Health Assembly to pass a 1987 resolution, which stated that HIV is "a naturally occurring [virus] of undetermined geographic origin". The HIV pandemic has challenged communities and brought about social changes throughout the world. Opinions on sexuality are more openly discussed. Advice on sexual practices and drug use – which were once taboo – is sponsored by many governments and their healthcare providers. Debates on the ethics of provision and cost of anti-retroviral drugs, particularly in poorer countries, have highlighted inequalities in healthcare and stimulated far-reaching legislative changes. In developing countries the impact of HIV/AIDS has been profound; key organisations such as healthcare, defense and civil services have been severely disrupted. Life expectancy has fallen. In Zimbabwe, for example, life expectancy was 79 years in 1991 but by 2001 it had fallen to 39 years.
Influenza
When influenza virus undergoes a genetic shift many humans have no immunity to the new strain, and if the population of susceptible individuals is high enough to maintain the chain of infection, pandemics occur. The genetic changes usually happen when different strains of the virus co-infect animals, particularly birds and swine. Although many viruses of vertebrates are restricted to one species, influenza virus is an exception. The last pandemic of the 19th century occurred in 1899 and resulted in the deaths of 250,000 people in Europe. The virus, which originated in Russia or Asia, was the first to be rapidly spread by people on trains and steamships. A new strain of the virus emerged in 1918, and the subsequent