There is no living being which lives everywhere. In this sense, all animal or vegetal species are specific to certain environmental conditions. In the world of parasites, being specific means being capable of exploiting a limited range of host species (the term “specific” can be also used to define the exploitation of certain organs or parts of organs, but this acceptance is less common).
Usually, a parasite species which infects a single species of host is called “oioxenic,” a parasite which infects a range of closely related host species is called “stenoxenic,” and a parasite which may develop in or on unrelated host species is called “euryxenic.” A general rule of parasitism is that specificity is narrow, with certain exceptions.
Human schistosomes, in their adult stages, provide examples for these different types of specificity.
Schistosoma haematobium, the agent of urinary schistosomiasis in Africa, is considered as strictly specific to humans, even if it has been occasionally...