Susceptible individual

In epidemiology a susceptible individual (sometimes known simply as a susceptible) is a member of a population who is at risk of becoming infected by a disease.

Susceptible individuals

Susceptibles have been exposed to neither the wild strain of the disease nor a vaccination against it, and thus have not developed immunity. Those individuals who have antibodies against an antigen associated with a particular infectious disease will not be susceptible, even if they did not produce the antibody themselves (for example, infants younger than six months who still have maternal antibodies passed through the placenta and from the colostrum, and adults who have had a recent injection of antibodies). However, these individuals soon return to the susceptible state as the antibodies are broken down.

Some individuals may have a natural resistance to a particular infectious disease. However, except in some special cases such as malaria, these individuals make up such a small proportion of the total population that they can be ignored for the purposes of modelling an epidemic.