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. The infection of a disease begins when a pathogenic (disease-causing) infectious agent, or a pathogen, is successfully transmitted from one
host to another. the host becomes infectious after the onset of symptoms. In this case, the latent period is longer than the incubation period. In these two cases, the disease can be effectively controlled using symptomatic surveillance. However, for some infectious diseases, the symptoms of the clinical disease may appear after the host becomes infectious. In this case, the pre-infectious or latent period has a shorter duration than the incubation period, the infectious period begins before the end of the incubation period and the host can infect others for some time without showing any noticeable symptoms. This early or mild stage of infection whose symptoms stay below the level of clinical detection is called
subclinical infection and the individual concerned is called an
asymptomatic carrier of the disease. For example, in HIV/AIDS, the incubation period lasts years longer than the latent period. So an HIV infected individual can show no symptoms and unwittingly infect other susceptible individuals for many years. In
COVID-19, the infectious period begins approximately 2 days before the onset of symptoms and 44% of the secondary infections may happen during this pre-symptomatic stage. In these kinds of cases with a significant number of pre-symptomatic (asymptomatic) transmissions, symptomatic surveillance-based disease control measures (such as isolation, contact tracing, enhanced hygiene, etc.) are likely to have their effectiveness reduced, because a significant portion of the transmission may take place before the onset of symptoms and this has to be taken into account when designing
control measures. The infectious period is a very important element in the infectious disease spreading dynamics. If the infectious period is long, then the measure of secondary infections (represented by the
basic reproduction number, R0) will generally be larger, regardless of the infectiousness of the disease. For example, even though HIV/AIDS has a very low transmission potential per sexual act, its basic reproduction number is still very high because of its unusually long infectious period spanning many years. From the viewpoint of controlling an epidemic, the goal is to reduce the effective infectious period either by treatment or by isolating the patient from the community. Sometimes a treatment can paradoxically increase the effective infectious period by preventing death through supportive care and thereby increasing the probability of infection of other individuals.
Generation time The
generation time (or
generation interval) of an infectious disease is the time interval between the beginning of infection in an individual (infector) to the time that person transmits to another individual (infectee). The generation time specifies how fast infections are spreading in the community with the passing of each generation. In contrast, the effective reproductive number determines in what number the infections are spreading in the community with the passing of each generation. The latent period and the infectious period helps determine the generation time of an infection. The mean generation time is equal to the sum of the mean latent period and one-half of the mean infectious period, given that infectiousness is evenly distributed across the infectious period. Since the precise moment of infection is very difficult and almost impossible to detect, the generation time is not properly observable for two successive hosts. Generally, in infectious disease statistics, the onset of clinical symptoms for all the hosts are reported. For two successive generations (or cases or hosts) in a chain of infection, the
serial interval is defined as the period of time between the onset of clinical symptoms in the first host (infector) and the onset of analogous clinical symptoms in the second host (infectee). Just like the generation time, the length of the serial interval depends on the lengths of the latent period, the infectious period and the incubation period. Therefore the serial interval is often used as a proxy measure to estimate the generation time. ==Usage of the term outside epidemiology==