susceptibles

Susceptibles refers to individuals or organisms that are not currently infected with a given pathogen but are at risk of infection because they lack immunity or other protective factors. In epidemiology, the susceptible population is the subset of a population without immunity to the disease in question. The size of this group depends on immunity levels, vaccination coverage, previous exposure, age, health status, and genetic or biological factors. Susceptibility can vary across individuals and over time; it can be reduced by vaccination or prior infection, and it can increase with waning immunity or new strains.

In mathematical models of infectious disease, such as the SIR framework, susceptibles are represented by the S compartment. The flow of individuals from S to I depends on the transmission rate, contact patterns, and the number of infectious individuals. Tracking susceptibles helps estimate the basic reproduction number and the potential for outbreaks. Public health interventions often aim to reduce susceptibility in the population through vaccination, prophylaxis, or behavioral measures, thereby decreasing the effective reproduction number and slowing or preventing transmission.

It is important to distinguish susceptibility from immunity or resistance. An individual may be susceptible but not exposed, or conversely may be exposed yet not infected due to partial immunity or genetic factors. Susceptibility is a dynamic property that can change over time with vaccination, new strains, or changes in health status.

See also: herd immunity, immunity, vaccination, epidemic modeling, SIR model.