Disease transmission is the process by which contagious diseases are spread from one organism to another. The major pathways through which infectious disease can be spread to an individual include direct or indirect contact; airborne transmission; food, water supply, or blood contamination; and vector-borne transmission. Vector-borne transmission occurs when the disease is not passed directly from person to person, but instead occurs when an infected organism—such as a parasite, insect, or animal—is responsible for carrying the disease and transmitting it to another host.

Plague is an example of a bacterial disease that is most commonly transmitted to humans through a vector—here, the bite of an infected flea. In rare cases, humans contract plague through direct contact with an infected animal carcass, or through inhaling droplets expelled when an infected person or animal coughs or sneezes. Malaria is another disease spread through a vector—in this case, the bite of a female Anopheles mosquito infected with a particular parasite.

Plague that enters the body through the skin is called bubonic plague. Far less common forms of plague occur when infection spreads to the lungs or poisons the blood. Bubonic plague is most readily transmitted in urban environments where poor sanitation, overcrowding, and a large population of rodents exist. When rodents become infected, fleas that feed on their blood can spread the disease to other animals they bite. Besides rodents, other animals that carry the bubonic plague bacteria, Yersinia pestis, include lice, dogs, cats, squirrels, and chipmunks. Once inside a human body, plague bacteria travel to the lymph nodes—an important component of the immune system—and begin to multiply. Within two to six days of exposure, plague symptoms develop. These include swollen and tender lymph glands, fever, chills, headache, and general weakness. If bubonic plague is left untreated, its mortality rate can be as high as 90 percent. If patients are diagnosed and treated early with antibiotics, the mortality rate can drop to 15 percent.

For centuries, plague devastated populations in Asia, Africa, and Europe. Today, human plague is far less prevalent than other infectious diseases. A few thousand cases of plague are reported each year worldwide, including an average of 10 to 20 cases in rural areas of the Southwestern United States. In these cases, transmission occurs primarily through flea bites from wild rodent species.

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