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.