Parasites can cause pain . Parasites can provoke adverse changes in host behavior . Parasites that feed on skin or live most of their lives in association with the integument of the host, thus producing an infestation , are known as ectoparasites . Parasites that dwell primarily in the internal tissues of the host, thus producing an infection , are known as endoparasites . Parasites and hosts are individuals living in large, complex populations, communities and ecosystems that include many other organisms.
Adaptation is the key to parasite success.
Success of the parasitic species depends on, among other things, the success of its various host species. Parasitism is full of risks for both the parasite and the host. If the host becomes extinct, the extremely host-specific parasite will probably disappear as well. A parasite that has adapted the ability to survive in several definitive hosts increases its chances for success, over evolutionary time, in at least one species. Survival of a parasite requires that the parasitic species must be able to spread from one susceptible host to another susceptible host.