In southern California, however, the fleas deserted the rats for ground squirrels, and one county in particular, Contra Costa County, had an epidemic which caused the squirrels to die by the thousands. The attention of the scientists was thus turned to the squirrel as a host of the flea, and a warfare similar to that against the rat has been for a year past carried on against the infected squirrels. Between September 24, 1908, and April 12, 1909, 4722 ground squirrels were killed and examined for plague infection, and from June 4 to August 13, 1909, the work being continued, 178 squirrels were found to have the plague.
Now that the relation between fleas and their hosts and the transmission of the disease is known, there need be but little fear in the future of this old enemy of man again getting control and spreading without hindrance throughout a whole country.
CHAPTER XX
DISEASES CONTROLLED BY ANTITOXINS (SMALLPOX, RABIES, TETANUS)
Smallpox.
A hundred years ago, the most dreaded disease in this country or in Europe was smallpox; and even yet writers of fiction, when they desire to expose their hero to the most harrowing conditions possible, leave him in a deserted hut with a man dying of smallpox. But to the educated person of to-day smallpox is encountered absolutely without dread, since it has been robbed of its terrors by the introduction of vaccination. As far back as 1717, Lady Mary Montague, writing home to England, described the eastern method of taking smallpox deliberately, under comparatively agreeable conditions, in order that severe cases of the disease might be prevented.
Why one attack of the disease should prevent a subsequent case was not known, nor why inoculation with other virus than that of the disease itself should be efficient was not known. But the fact was thoroughly established then that in some way, in the process of the disease and recovery, there was left in the body some substance or agency which was sufficiently powerful to ward off subsequent attacks.
In 1796, Dr. Jenner discovered that a disease very similar to smallpox existed in the cow, and that if the scab from a pustule on the cow was used for inoculation instead of similar material from a smallpox patient, the resulting disease would be less severe and the protection against subsequent attacks equally efficient. Since that time, therefore, cowpox matter or vaccine has been used to develop a mild form of disease for the express purpose of preventing subsequent attacks.