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What is plague?

Plague is a bacterial disease that can be fatal if left untreated. There are three forms of the disease:

  • Bubonic plague, in which lymph nodes close to the site of infection become swollen and painful
  • Septicemic plague, in which the bacteria spreads throughout the body in the bloodstream
  • Pneumonic plague, in which the lungs become infected. Pneumonic plague can be transmitted directly from one patient to another through coughing.
A grey and light brown squirrel stands partially emerged from a hole in the ground.
Ground squirrels quickly die when infected with plague.

Plague is thought to have originated in Asia and to have spread throughout Europe through the movement of trade ships. It is the agent of multiple pandemics, including the plague of Justinian in the 6th century and the Black Death in the 14th century. Plague was detected in San Francisco in 1900, and through intensive public health efforts, the City was declared plague-free in 1908.

In modern time, plague still exists amongst wild rodents in California. The bacteria are transmitted through flea bites. Some species, like ground squirrels, are rapidly killed by the disease. Other species survive but may remain infected for a period of time. Humans become sick when they are exposed to infected fleas, infected animals or animal carcasses, or infected pets. There are case studies of domestic cats infecting their owners through sneezing or coughing.

Plague fact sheet

What are the symptoms of plague?

Plague in humans is relatively rare and can be treated successfully with modern antibiotics. Symptoms depend on the type of plague; however, nearly all forms create early symptoms of fever (over 100 F), chills, headache, and weakness. Learn more about plague signs and symptoms HERE. It is vital that plague be recognized and treated in its early stages. If not, it is often fatal. If you are experiencing any of these symptoms, seek immediate medical attention. Pneumonic plague can be transmitted from person to person.

Is there plague in San Mateo County?

Yes, plague has been documented in San Mateo County. Plague was detected on San Bruno Mountain in the 1940s and is detected sporadically from wild rodents collected from the northern Santa Cruz mountains since 1975. There have been no major outbreaks detected in the past 20 years.

To learn more about plague in California, visit the California Department of Public Health webpage HERE.

Sideview of a light brown colored flea.  The flea has 6 legs, including a very long pair of back legs. The flea appears to have small hairs occurring spottily over its body.
The Oriental rat flea is an important vector of plague. This image is from the National Center for Veterinary Parasitology.

How can I get plague?

  1. Bites of fleas from infected rodents.
  2. Direct contact with sick animals.
  3. Transmission from infected pets. Infected fleas can be brought into the home or campsite by a pet. Infected pets can transmit plague when sneezing or coughing.

Learn more about plague transmission HERE.

What is the risk of becoming infected with plague in San Mateo County?

The risk of being infected with plague in San Mateo County is very low. In California, the majority of plague detections occur in rural to semi-rural areas of the foothills and mountainous areas. Most human cases occur after exposure to fleas following a significant outbreak in a wild rodent population.

How can I protect myself against plague?

You can minimize your exposure to plague by carefully following the precautions listed here:

  • Avoid all contact with wild rodents and their fleas
  • Report dead wild rodents (particularly ground squirrels and rabbits) to the District
  • Consult a veterinarian if your pet is sneezing or coughing and has been outside in a rural or semi-rural area
  • Keep rodent populations down around homes and conduct exclusion work to keep rodents out of buildings.
  • Protect pets against fleas by keeping them indoors or treating them with flea prevention - talk with your pet's veterinarian about the the best flea prevention options for your pet.


Page last reviewed: December 28, 2023

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