|Description:||From a date prior to 1967, this historic photograph depicted three scientists extracting a blood specimen from a rabbit during a plague investigation.|
Plague is caused by a bacterium called Yersinia pestis, and is transmitted from rodent to rodent by infected fleas. At the time, these men were staff members at the former San Francisco, California, Public Health Service, Communicable Disease Center, Plague Laboratory.
The Communicable Disease Center (CDC) was a branch of the U.S. Public Health Service (PHS), which was a branch of the former Department of Health, Education, and Welfare (HEW). Retaining its acronym, in 1970 the CDC was renamed to the Center for Disease Control, and in 1979, HEW was renamed the Department of Health and Human Services.
Rock squirrels and their fleas are the most frequent sources of human infection in the southwestern states. For the Pacific states, the California ground squirrel and its fleas are the most common source. Many other rodent species, for instance, prairie dogs, wood rats, chipmunks, and other ground squirrels and their fleas, suffer plague outbreaks and some of these occasionally serve as sources of human infection. Deer mice and voles are thought to maintain the disease in animal populations but are less important as sources of human infection. Other less frequent sources of infection include wild rabbits, and wild carnivores that pick up their infections from wild rodent outbreaks. Domestic cats (and sometimes dogs) are readily infected by fleas or from eating infected wild rodents. Cats may serve as a source of infection to persons exposed to them. Pets may also bring plague-infected fleas into the home.