What's so special about within-host dynamics?

When an epidemiology conference hosts a session on ‘within-host dynamics’, three questions immediately come to a discussant's mind: ‘Why are we doing this?’, ‘What are we doing here?’ and ‘What difference does being within a host make?’ The first of these three questions is answered by the quality of the papers presented in this session. There are many fascinating questions about the pathogenesis of infectious diseases, and about the dynamics of host responses to infectious organisms. These questions often involve highly nonlinear interactions between host and pathogen within the host organism. The rigour and clarity of thought required by mathematical description of such interactions is a great aid in developing an intuitive understanding of which processes are important, and of what patterns those processes might generate.

The subject matter of the four talks: two on HIV, one on malaria and one on schistosomiasis is probably a fair representation of the field. The enigma of HIV's pathogenesis has prompted many theoretical (and empirical) investigations. Nowak's theory is one elegant example of the numerous theories proposed to explain the long period between infection with HIV and illness with AIDS (reviewed in McLean 1993). In contrast to the care and rigour with which Nowak's theory has been expounded, some of the ‘verbal theories’ of HIV's pathogenesis are classic examples of why biologists ought to make mathematical models; so that they can see when the predictions made by their verbal models simply cannot be matched up with the patterns they aim to explain. A cogent argument for the use of mathematical models in an exploratory fashion by biologists is given by Hillis (1993).