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 alive by transmission from animal to animal, and the persistence of the infection is favored by mild and chronic cases.

CHAPTER IX

DISEASE CARRIERS.—­THE RELATION BETWEEN SPORADIC CASES OF INFECTIOUS DISEASE AND EPIDEMICS.—­SMALLPOX.—­CEREBRO-SPINAL MENINGITIS.—­POLIOMYELITIS.—­VARIATION IN THE SUSCEPTIBILITY OF INDIVIDUALS.—­CONDITIONS WHICH MAY INFLUENCE SUSCEPTIBILITY.—­RACIAL SUSCEPTIBILITY.—­INFLUENCE OF AGE AND SEX.—­OCCUPATION AND ENVIRONMENT.—­THE AGE PERIOD OF INFECTIOUS DISEASES.