The reasons for the biological progress of parasites.

As a group, parasites are unusually diverse. Even closely related parasites can behave differently, infecting different types of hosts, causing different pathologies, or infecting different tissues. This diversity is based on evolution. It is widely accepted that parasites are prone to rapid evolution, and because of their often short generation times and large population sizes, parasites can develop much faster than their hosts. Efforts to understand the evolution of the parasite and the relevance of this evolution to disease for at least half a century have returned to the first observations of the development of drug resistance in bacteria.