This complex life cycle can follow a number of alternative paths. These are parasites of mammal. The parasitic stage is exclusively parthenogenetic females, which live in the intestine. The females produce eggs that pass down in gut and hatch to release a stumpy, "rhabditiform" first larval stage. In some circumstances (such as in an immunocompromised host) the L1 can molt twice while within the host to produce a filariform L3. These long, slender filariform L3s are capable of infecting the host, so in this case they autoinfect the host: they penetrate the gut, enter the bloodstream, pass through heart, then lungs, then into the alveoli. Finally they move up the bronchi, enter the digestive system, and mature into another generation of parasitic, parthenogenetic female.
Larvae that are passed with the feces develop along one of two paths. In the heterogonic path, the rhabditiform L1 molts twice to form rhabditiform L3 of both sexes. These become adults, mate and produce eggs. The eggs can produce larve that also develop in this free-living, heterogonic path for an indefinite number of cycles. The homogonic path starts with rhabditiform L1s. It does not matter whether these hatch from eggs produced by a parasitic or a free-living female. Some develop into filariform L3s. These ones must infect a host, and can do so by two routes. If ingested, the L3 may pass directly into the intestine, molt twice, and become parthenogenetic females. The L3 may also penetrate the skin, and migrate either via the bloodstream-heart-lung route, or directly through tissues, to reach the intestine, where they mature into partheonogenetic females.