Models are frequently used by epidemiologists both to analyse and to predict the spread of infectious diseases. This is important both socially and economically, in addition to being of academic interest. The graph of influenza cases in Canada 1996-97 shows the pattern of an influenza epidemic over winter and into spring. Why does it take this form? Is it predictable? Models can provide valuable insights into such patterns.
Kilbourne’s model charts the progress of a particular infectious disease over a period of time (a 'chronological study'), showing how the different variables can change. At first the infection rate increases. Since those who have recovered from the disease are immune, having acquired antibodies that fight the infection, the size of the susceptible population gradually decreases. Eventually the wave of infection subsides.
Complications arise when a virus changes its structure ('antigenic shift'), thus causing another wave of infection. The whole population is again susceptible, having no antibodies to the new structure.