The actual mortality cannot be known, but has been
estimated at fifty millions. Plague played a large part in the
epidemics of the Middle Ages. An epidemic started in 1346 and had as
great an extension as the Justinian plague, destroying a fourth of the
inhabitants of the places attacked; and during the fifteenth and
sixteenth and seventeenth centuries the disease repeatedly raised its
head, producing smaller and greater epidemics, the best known of
which, from the wonderful description of De Foe, is that of London in
1665, and called the Black Death. Little was heard of the disease in
the nineteenth century, although its existence in Asia was known. In
1894 it appeared in Hong Kong, extended to Canton, thence to India,
Japan, San Francisco, Mexico, and, in fact, few parts of the tropics
or temperate regions of the earth have been free from it. Mortality
has varied greatly, being greatest in China and in India; in the last
the estimate since 1900 is seven million five hundred thousand deaths.
The disease is caused by a small bacillus discovered in 1894 which
forms no spores and is easily destroyed by sunlight, but in the dark
is capable of living with undiminished virulence for an indefinite
time. The disease in man appears in two forms, the most common known
as bubonic plague, from the great enlargement of the lymph nodes,
those of the groin being most frequently affected. The more fatal form
is known as pneumonic plague, and in this the lungs are the seat of
the disease.

In the old descriptions of the disease it was frequently mentioned
that large numbers of dead rats were found when it was prevalent, and
the most striking fact of the recent investigations is the
demonstration that the infection in man is due to transference of the
bacillus from infected rats. There are endemic foci of the disease
where it exists in animals, the present epidemic having started from
such a focus in Northern China, in which region the _Tarabagan_,
a small fur-bearing animal of the squirrel species, was infected. Rats
are easily infected, the close social habits of the animal, the vermin
which they harbor, and the habits of devouring their dead fellows
favor the extension of infection. The disease extends from the rat to
man chiefly by means of the fleas which contain the bacilli, and in
cases of pneumonic plague from man to man by means of sputum
infection. The disease once established in animals tends to remain,
the virus being kept