Parasites that use intermediate and paratenic hosts have indirect life cycles . Parasites succeed, biologically-speaking, by surviving and reproducing. In a complex ecosystem, some of these randomly-distributed parasite transmission stages may take paths that will not lead to success. Many transmission stages succumb to unsuitable environmental extremes of temperature, humidity, exposure to sunlight, etc . Parasites may enter aberrant hosts , in whom conditions are not adequate for survival, further development, or sexual maturity.
Aberrant hosts are a dead-end for both the individual parasite and the parasite species.
Almost every occurrence of a parasite in an aberrant host results in the failure of aberrant parasites to either survive or to produce viable transmission stages. These are a few of the most basic concepts of parasitology. There are exceptions to any rules, and the definitions used in parasitology are constantly tested by new discoveries about parasites, hosts, and parasitic relationships.