Like all living species, parasites have adapted to particular  environmental conditions. No parasite species exists everywhere on earth. For a parasite to exist at any one place, there needs to be a suitable host and a suitable environment for transmission.

The expression “suitable host” implies several constraints: not only must the host be “qualitatively” present (i.e., a susceptible population), but also quantitatively (i.e., a density of hosts allowing sufficient production of larval stages and transmission); in heteroxenous life cycles, the constraints are multiplied by the number of different host species necessary for the completion of the cycle.

The expression “suitable environment for transmission” implies other constraints, because (with very few exceptions like Trichinella), parasites spend a part of their life outside the hosts and have, like all organisms, particular requirements such as range of temperature, presence of water, etc.