The parasitic roundworm called Strongyloides stercoralis mainly infects humans. This parasite has different types of life cycles. One is direct, similar to that of the hookworm. It is transmitted in the following manner:
After a short feeding period and development in the soil, the larvae penetrate human skin, enter the blood stream, and pass through the right side of the heart to the lungs. From the lungs, the adolescent parasites go up the windpipe into the mouth, are swallowed, and reach the upper part of the small intestine where they develop into mature worms. Under certain conditions, parasites may undergo an indirect life cycle in which free-living mature male and female worms develop in the soil and produce a new generation of large numbers of larvae.
At times, the larvae may develop rapidly into the infective state in the intestine where they penetrate the intestinal lining instead of passing out of the body in the feces, as occurs normally. This modification of the life cycle, called internal autoinfection, explains persistent strongyloidiasis, as long as 40 years, in patients who have moved to areas where the disease is not generally found. Autoinfection may produce heavy infections and severe disease, especially in patients with reduced immunity such as those receiving corticosteroids or other immunosuppressive drug treatment.

This infection is diagnosed by laboratory investigations including examination of feces and duodenal contents for larvae. A reliable blood test has also been developed in the US to detect antibodies to Strongyloides.