The infecting organism _pyrosoma bigenimum_ is a tiny pear-shaped parasite of the red corpuscles.

Smith's investigations on the disease, published in 1893, is one of the classics in medicine, and one of the few examples of an investigation which has not been changed or added to by further work.
One of the most interesting methods of extension of infection, showing on what small circumstances infection may depend, is seen in the case of the hookworm disease, which causes such devastation in the Southern States.

The organism which produces the disease, the _Uncinaria_, belongs to the more highly developed parasites, and is a small round worm one-third of an inch long.

The worms which inhabit the intestines have a sharp biting mouth by which they fasten themselves to the mucous membrane and devour the blood.

The most prominent symptom of the disease is anaemia, or loss of blood, due not only to the direct eating of the parasite, but to bleeding from the small wounds caused by its bite.