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Many parasites infect multiple host species, with one host species being the definitive host (where the parasite reproduces), and the other host species being the intermediate host (where the parasite grows to maturity). (Further details described as "complex life cycle" at https://en.wikipedia.org/wiki/Parasitic_life_cycle ) Why is this complex life cycle so common? Why wouldn't more parasites adopt a simpler life cycle of just one host?

I knew that there were some parasites that infected more than one kind of host species, but now that I am looking into it, it seems like a majority of internal parasites have this kind of life cycle. Does anyone know why?

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I assume that by internal parasites you mean animals like tapeworms etc?

They can infect several species simply because they are not picky. These parasites do not rely on precise molecular hijacking mechanisms like intracellular parasites (e.g. Apicomplexa).

All they need is a certain amount of heat and humidity (which any host provides) and food pre-digested by the host, which is essentially the same for host species that share the same feeding habits, so there is no reason a priori for them to have a narrow range of hosts.

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  • $\begingroup$ I've rewritten the question to make it more clear. I am interested in why there is such a common pattern of having a specific complex life cycle of definitive and intermediate hosts. I was thinking this looked more common in what I call "internal" parasites (i.e. endoparasites, like tapeworms, protozoa, etc.), but the question really goes for any parasite with that complex life cycle. (P.S. I apologize for the delay. I guess I shouldn't have posted the question just prior to traveling for Thanksgiving.) $\endgroup$ – Greg Boettcher Nov 22 '18 at 10:36
  • $\begingroup$ This answer does not have any references, misses the point of the question, and suggests lack of familiarity with parasitology. Many multicellular internal parasites have specific host tropism, corresponding to specific life cycle stages. $\endgroup$ – De Novo Dec 23 '18 at 1:43

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