Strongyloidiasis, or Cochin-China diarrhea, is caused by a minute nematode, the threadworm Strongyloides stercoralis. The organism was first discovered in 1876 in French troops who had suffered severe diarrhea in the Cochin-China region of Vietnam. Strongyloides occurs around the world, with a range similar to that of the hookworms. Millions of people harbor the organism. Because poor sanitation and going without shoes favor transmission, it is especially prevalent in poor tropical countries. Like hookworm disease, strongyloidiasis prevalence has declined greatly in the southern United States since the early twentieth century, but it still exists in foci in Kentucky and other states.

The worm has a complex life cycle. Parasitic males may not exist, but if they do, they are eliminated from the body shortly after infection. The females burrow in the mucosa of the intestine, where they feed and lay their eggs, apparently by parthenogenesis. The eggs pass into the lumen of the intestine, where they hatch into a rhabditiform larval stage. In most cases these larvae are voided in the feces to the soil and either transform themselves directly into an infective filariform larval stage, or, if conditions are favorable, undergo one or more generations of sexual reproduction before filariform larvae appear. Like the hookworms, the filariform Strongyloides larvae penetrate human skin, often on an unshod foot, enter the venous circulation, and are carried through the heart to the lungs. Here they burrow through the walls of the air sacs, ascend to the throat, and are swallowed. The adult females bore into the walls of the small intestine and sometimes into the wall of the large intestine as well.