In relation to many directly transmitted infections, the starting point is often the "compartmental model" (slide). According to this, all individuals of a population belong to one of three categories or "compartments": a/ susceptible, b/ infected and infectious, and c/ recovered and immune.

As models depict the most essential elements of the spread of an infection, several assumptions must be made. The most important ones usually are: a/ the latent period (from the point of infection to the beginning of the infectious period) is negligible, b/ the duration of infectivity is identical to that of clinical disease, c/ after recovery everyone becomes immune and stays immune, d/ there is no degree of infectiousness (people are either infectious or not), e/ the population is closed, i.e. no one enters (births, immigration) or leaves (deaths, emigration), f/ everyone in the population has the same chances to meet everyone else (homogeneous mixing).