ABSTRACT
It is often necessary to allow for different types of individual in infectious disease models. For example, our model needs to keep track of elderly infectives if, as for influenza, they are known to be at greater risk of severe illness. Models also need to allow for different types of individual when there are identifiable groups who play different roles in the transmission of the infection. Such differences can have a biological source. For example, a less developed immune system in children may make them more susceptible to infection or, when infected, have a longer infectious period than infected adults. Different roles in transmission can also have a social source. For example, the frequency and types of contact schoolchildren have differs from those of adults.
