Parasitism is the most popular lifestyle on Earth. Roughly, half of plants and animal species are parasitic at some stage of their life cycle. Few species, if any, lack any parasites and most species have at least one host-specific parasite species. Even parasites have parasites. We know the most about humans, who host 342 parasite species, not including viruses and bacteria. Many of these only parasitize humans. Parasites tend to be more abundant in places where hosts are abundant and more diverse in places where hosts are diverse. For instance, more parasites species are found in the tropics than at high latitudes. While parasite diversity follows host diversity, parasitism may also select for diversity in host communities.
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