The evolution of virulence has 3 different modes and hypotheses. The first is coincidental mode. This hypothesis says that the virulence is not the target of selection but it is a coincidence or by product of selection on other traits. The second is the short-sighted hypothesis. In this hypothesis the fitness in the host is increased but the ability of transmission to a new host is less likely. The third hypothesis is the trade-off. This hypothesis is where there is a trade-off between killing their host and also increasing their ability of transmission.
There are many examples of the trade-off hypothesis but the book shows one example of a pathogenic fungus that kills an insect and sprouts from it. I found this interesting for a fungus to kill its host but it is actuallly quite common. An interesting exmple is the zombie carpenter ant that has fallen victim to the fungus in the family Cordyceps. The fungus releases chemicals that exert a mind control over the ant. The ant climbs a plant and latches it mandibles at the stem of the leaf. This becomes the ants resting place, because the stalk of the fungus sprouts out of the ants head. The fungus kills its host but in the process it increases its potential of transmission by exerting its mind control.