Book 5 now and at about 100 pages not too bad either.
It considers some of the theoretical basis of disease. The arms race between pathogen and the host and how they both evolve over time. It also looks at things like the manipulation of the host, particularly by some pathogens that can have a quite large effect on the behaviour of the host. This also considers the patterns of virulence and how the life of the pathogen is affected by the life of the host.
Transmission mechanisms also are looked at in detail, and how effective they are.
Finally in the last chapter it looks at a theoretical model for a host/pathogen infection in a population and how that can be modelled. This is apparently setting us up nicely for the next book on epidemiology.