low fever would have been impossible;
for it could easily have been shown that the ordinary _culex_
mosquito played no role. The role which insects may play in the
transmission of disease was first shown by Theobald Smith in this
country, in the transmission by a tick of the disease of cattle known
as Texas fever. 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. Large numbers of eggs are produced by the parasite which
are passed out with the feces, which becomes the only infectious
material. In a city provided with water-closets and a system of
sewerage there would be no means of extension of infection. The eggs
in the feces in conditions of warmth and moisture develop into small
crawling larvae which can penetrate the skin, producing inflammation of
this, known in the region as the ground itch. The larvae enter the
circulation and are carried to the lungs, where they perforate the
capillaries and reach the inner surface; from this they pass along the
windpipe, and then by way of the gullet and stomach reach their
habitat, the small intestine. Unfortunately, the habits and poverty of
the people in every way facilitate the extension of the infection.
There is no proper disposal of the feces, few of the houses have even
a privy attached to them, and the feces are distributed in the
vicinity of the houses. This leads to contamination of the soil over
wide areas. Most of the inhabitants of the country go barefoot the
greater part of the year, and this gives ready means of contact with
the la