How Fleas Work

Fleas and Disease

Fleas can spread disease. These tiny parasites are actually to blame for spreading bubonic plague. Carol Yepes/Getty Images

Most people think of fleas as a nuisance — they bite people and animals, causing itchy, red bumps. This reaction comes from substances in fleas' saliva, and some people and animals react to these more strongly than others. Fleas often draw blood from two or three tightly spaced punctures, causing two or three itchy bumps.

Animals that develop a sensitivity to flea bites can scratch or groom themselves excessively. This can lead to rough skin, baldness and skin infections. Sometimes, these infections can be mistaken for a parasitic infection called mange. Itchy, uncomfortable pets also can be ill-tempered or restless.


In addition to being annoying, fleas can transmit diseases to other parasites, people and animals. Flea larvae feed on the eggs of tapeworms. Once swallowed, the tapeworms begin to develop in the gut of the flea. If an animal swallows an infected flea while grooming, the animal also becomes infected with the tapeworm.

The tapeworm grows in the intestine of the animal, shedding its egg sacs from the animal's rectum as it sleeps. The sacs land in areas near the animal's bedding — where flea larvae are likely to live — and the cycle starts all over again. People, especially children, also can become infected with tapeworms if they swallow fleas while petting animals, or if they come in contact with infected waste and don't wash their hands before eating.

Fleas also can transmit serious diseases, such as the bubonic plague. Fleas are vectors, which means they transport plague bacteria from rodents — the natural carriers of the disease — to people. A particular species of flea, the Oriental rat flea, usually is the culprit. In addition to preferring to feed on rats, the Oriental rat flea's gut can become blocked by plague bacteria. Then, when it bites its next host, it may regurgitate infected blood into the wound. Symptoms of plague include swollen lymph nodes called buboes, fever, headache and exhaustion.

Fleas also can carry murine typhus, particularly in the Southern and Southwestern parts of the U.S. Murine typhus is caused by the bacteria Rickettsia typhi. The Oriental rat flea and cat flea can carry these bacteria to people. The infection typically comes from the flea's waste rather than its mouth. The fleas defecate while they're eating, and their hosts scratch the infected waste into the bite. People also may break the skin while scratching, and the bacteria can enter the body through the scratch. Murine typhus causes symptoms like fever, headaches and nausea. Both it and bubonic plague are treated with antibiotics.