There is no parasite that is universally infective, even generalist parasites that can infect many different host species are usually limited to a particular taxonomic group - such as fish, insects, or mammals. Some parasites may infect a broad spectrum of hosts during one stage of their life-cycle, but are very specific in another. For parasites that are host specialists, this can be taken to an extreme level where they are found exclusively on just one particular host species. Just how parasites evolve from generalist to become so specialised is one of the enduring questions in studies of the evolutionary ecology of parasites.
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