An organism that lives throughout a major period of its life in or on another single host individual, deriving its food from the host and causing lowered survival or reproduction of the host. 

Distinguishing features of parasites: 

  • parasites live in intimate association in or on its host
  • association lasts for a major portion of a parasites' life
  • all of the parasites food is derived from one host
  • death of host prior to a stage of a parasites' development results in the death of the parasite
For coevolution to proceed there must be mutual evolution: the host evolves defense mechanisms while the parasite evolves mechanisms to neutralize these defenses. 
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