In epidemiology, particularly in the discussion of infectious disease dynamics (modeling), the latent period (also known as the latency period or the pre-infectious period) is the time interval between when an individual or host is infected by a pathogen and when that individual becomes infectious, i.e. capable of transmitting pathogens to other susceptible individuals.
Relationship with related concepts in infectious disease dynamics To understand the spreading dynamics of an infectious disease or an epidemic, three important time periods should be carefully distinguished: incubation period, pre-infectious or latent period and infectious period. Two other relevant and important time period concepts are generation time and serial interval. The infection of a disease begins when a pathogenic (disease-causing) infectious agent, or a pathogen, is successfully transmitted from one host to another. Pathogens leave the body of one host through a portal of exit, are carried by some mode of transmission and after coming into contact (exposure) with a new susceptible host, they enter the host's body through an appropriate portal of entry. Upon entering the new host, they take a period of time to overcome or evade the immune response of the body and to multiply or replicate after having traveled to their favored sites within the host’s body (tissue invasion and tropism). When the pathogens become sufficiently numerous and toxic to cause damage to the body, the host begins to display symptoms of a clinical disease (i.e. the host becomes symptomatic).
Incubation period The time interval from the time of invasion by an infectious pathogen to the time of onset (first appearance) of symptoms of the disease in question is called the incubation period. After the incubation period is over, the host enters the symptomatic period. Moreover, at a certain point in time after infection, the host becomes capable of transmitting pathogens to others, i.e. they become infectious or communicable. Depending on the disease, the host individual may or may not be infectious during the incubation period. The incubation period is important in the dynamics of disease transmission because it determines the time of case detection relative to the time of infection. This helps in the evaluation of the outcomes of control measures based on symptomatic surveillance. The incubation period is also useful to count the number of infected people. The period from the time of infection to the time of becoming infectious is called the pre-infectious period or the latent period. During the pre-infectious or latent period, a host may or may not show symptoms (i.e. the incubation period may or may not be over), but in both cases, the host is not capable of infecting other hosts i.e. transmitting pathogens to other hosts. The latent period, rather than the incubation period, has more influence on the spreading dynamics of an infectious disease or epidemic.
Infectious period The time interval during which the host is infectious, i.e. the pathogens can be transmitted directly or indirectly from the infected host to another individual, is called the infectious period (or the period of communicability), defined as the period from the end of the pre-infectious period or the latent period until the time when the host can no longer transmit the infection to other individuals. During the infectious period, a host may or may not show symptoms, but they are capable of infecting other individuals. The duration of the infectious period depends on the ability of the infected host individual to mount an immune response.
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