The following outline is provided as an overview of and topical guide to concepts related to infectious diseases in humans. Infection – Subsequent to their transmission via environment and evasion or overcoming of defense, the entry/invasion, establishment, and replication of disease-causing microscopic organisms (pathogens or agents) inside a host organism, and the reaction of host tissues to them and to the toxins they produce. Infectious disease – illness or disorder when pathogenic (disease-causing) microorganisms, such as bacteria, viruses, fungi, or parasites invade and multiply within the body of a host organism and release toxins, causing various clinical symptoms which can potentially lead to severe health complications or even death. Infectious diseases can be spread, directly or indirectly, from person to person, from animal to animal, or from animals to humans (zoonotic diseases), or through environmental exposure. They can be treated with medical interventions or prevented before they happen.
Determinants
Agent Infectious agent (also known as pathogen) – a biological entity (microscopic or macroscopic) that causes disease or illness in its host by invading and multiplying within the host's body, disrupting its normal physiological functions and leading to clinical symptoms (i.e. an infectious disease).
Pathogens and related disciplines There are mainly seven types of pathogenic microorganisms: viruses, bacteria, fungi, protozoa, helminths, other parasites and abnormal proteins called prions.
Host Host – a living organism, typically a human or animal, that harbors a pathogenic microorganism, providing the environment and resources necessary for it to survive, grow, and potentially reproduce or multiply. There is a symbiotic relationship between the two organisms, which can be commensal (mutually beneficial), but in the case of infectious diseases, harmful for the host.
Environment Environment – the external conditions and factors such as climate, sanitation, human behavior and living conditions that influence the spread, survival, and transmission of pathogens and the risk of infectious diseases.
Transmission Transmission – the process by which an infectious agent is spread from one host (an infected person) or environment (a reservoir) to another susceptible host person, facilitating the spread of disease.
Basic concepts
Modes Transmission mode – the mechanism through which an infectious agent travels from a source (an infected host or a reservoir) to a new susceptible host.
Endogenous Endogenous transmission – the spread of infectious agents within a single host organism, typically from their normal habitat in one part of the body to a new location within the body, rather than between individuals.
Endogenization – the evolutionary process by which viral genetic material becomes stably integrated into the germline of a host organism and inherited by offspring through Mendelian inheritance. Endogenous overgrowth – Overproliferation of the body's normal microbiota due to disruptions (e.g., antibiotics, immunosuppression), leading to infections (e.g., Clostridioides difficile colitis). Normal flora overgrowth – Disruption of the normal microbial balance leading to excessive growth of normally harmless commensal microorganisms in their native environment, causing opportunistic infections (e.g., bacterial vaginosis). Endogenous reactivation – Re-emergence of dormant pathogens due to immune suppression or stress (e.g., reactivation of latent tuberculosis or herpes viruses). Microbial translocation – Movement of microbes or their products (e.g., endotoxins) from the gut lumen into systemic circulation, often due to gut barrier dysfunction / permeability (e.g., in sepsis or liver cirrhosis). Endogenous seeding – Spread of pathogens from a primary site of infection to distant sites via the bloodstream or lymphatic system, leading to secondary infections (e.g., Staphylococcus aureus bacteremia causing metastatic abscesses). Biofilm formation – Microbes form protective, adherent communities on surfaces (e.g., catheters or tissues), enhancing resistance to antibiotics and immune responses, making infections chronic and difficult to treat (e.g., Pseudomonas aeruginosa in cystic fibrosis)
Exogenous Exogenous transmission – the transfer and spread of infectious agents between different individuals or from an external source to a host organism.
Cross-species Cross-species transmission (or host jump) – the transmission of infectious agents from one species (original animal reservoir) to a new one, potentially leading to infections in the recipient species.
Spillover infection – cross-species transmission of pathogens from a domestic or wildlife animal reservoir to a new human host. Vector – organism, typically an insect or arachnid, that transmits pathogens from an infected host to a susceptible host individual. Zoonosis – infectious disease transmissible from animals to humans. Reverse zoonosis – transmission of pathogens from humans to infect animals.
Human-to-human / Cross-infection Human-to-human transmission (also known as cross-infection) – direct or indirect transmission of infectious agents from one individual to another, leading to the spread of disease within a population.
Environment-to-human Sapronosis
Routes Routes of transmission – specific pathways through taken by infectious agents to travel from one infected host or reservoir to a new host.
Modelling Mathematical modelling of infectious disease – mathematical equations, tools and simulations to represent the dynamics of disease transmission within a population in order to understand and predict them for disease control and public health decision-making.
Occurrence in population
Anatomical location The specific area or organ within the body where pathogenic microorganisms invade and proliferate, leading to localized or systemic disease.
Prevention and control measures Infection prevention and control – strategies, measures and practices designed and implemented to prevent the spread of infectious diseases within healthcare settings and communities, reducing the risk of infections among patients, healthcare workers, and the general population.
Pharmaceutical Pharmaceutical prevention and control – Use of medications, vaccines, and other pharmaceutical interventions to prevent the spread of infectious diseases, treat infected individuals, and manage outbreaks within populations.
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