Over time, some diseases gradually become less harmful; others become more harmful. Much of what happens depends on the extent to which a parasite relies on a host for its own survival. If a parasite is not dependent on its current host for its survival, it can often afford to damage its host. And if, by damaging its host, it increases its fecundity, it will probably evolve to be more dangerous.
However, an extraordinary array of evolutionary trajectories are possible. For instance, although parasites that are only transmitted vertically ­ that is, from parent to child ­ usually evolve to become harmless, this is not the only outcome. For example, bacteria known as Wolbachia infect many insect species. They can only be passed vertically through the mother ­ so while some Wolbachia are indeed harmless, others alter the sex ratios of the insects’ offspring, so that more females are produced than males.
