rvae which crawl over the surface of the ground. The disease is
necessarily associated with poverty and ignorance, the amount of blood
is reduced to a low point, and industry, energy and ambition fall with
the blood reduction; the schools are few and inefficient; the children
are backward, for no child can learn whose brain cells receive but a
small proportion of the necessary oxygen; and a general condition of
apathy and hopelessness prevails in the effected communities. The
control of the disease depends upon the disinfection of the feces, or
at least their disposal in some hygienic method, the wearing of shoes,
and the better education of the people, all of which conditions seem
almost hopeless of attainment. The infection is also extended by means
of the negroes who harbor the parasite, but who have acquired a high
degree of immunity to its effects and whose hygienic habits are even
worse than those of the whites. The organism was probably imported
with the negroes from Africa and is one of the legacies of slavery.

The diseases of animals are in many ways closely linked with those of
man. In the case of the larger parasites, such as the tapeworms and
the trichina, there is a direct interchange of disease with animals,
certain phases of the life cycle of the organisms are passed in man
and others in various of the domestic animals. A small inconspicuous
tapeworm inhabits the intestine of dogs and seems to produce no ill
effects. The eggs are passed from the dog, taken into man, and result
in the formation of large cystic tumors which not infrequently cause
death. Where the companionship between dog and man is very close, as
in Iceland, the cases are numerous.

Most of the diseases in animals caused by bacteria and protozoa are
not transmitted to man, but there is a conspicuous exception. Plague
is now recognized as essentially an animal disease affecting rats and
other small rodents, and from these the disease from time to time
makes excursions to the human family with dire results. The greatest
epidemics of which we have any knowledge are of plague. In the time of
Justinian, 542 B.C., a great epidemic of plague extended over what was
then regarded as the inhabited earth. This pandemic lasted for fifty
years, the disease disappeared and appeared again in many places and
caused frightful destruction of life. Cities were depopulated, the
land in many places reverted to a wilderness, and the works of man
disappeared.