We now turn to a study of the population biology of disease. We will consider both microparasites – in which populations increase in hosts by multiplication of numbers – and macroparasites – in which populations increase in hosts by both multiplication of numbers and by growth of individual disease organisms. The age of genomics and bioinformatics makes the material in this chapter more, and not less, relevant for three reasons. First, with our increasing ability to understand type and mechanism at a molecular level, we are able to create models with a previously unprecedented accuracy. Second, although biomedical science has provided spectacular success in dealing with disease, failure of that science can often be linked to ignoring or misunderstanding aspects of evolution, ecology and behavior (Schrag and Weiner 1995, de Roode and Read 2003). Third, there are situations, as is well known for AIDS but is true even for flu (Earn et al. 2002), in which ecological and evolutionary time scales overlap with medical time scales for treatment (Galvani 2003).

To begin, a few comments and caveats. At a meeting of the (San Francisco) Bay Delta Modeling Forum in September 2004, my colleague John Williams read the following quotation from the famous American jurist Oliver Wendell Holmes: “I would not give a fig for simplicity this side of complexity, but I would give my life for simplicity on the other side of complexity”.