In the United States, Lyme disease is caused by the bacterium Borrelia burgdorferi, which normally lives in animals such as mice, squirrels, and some birds. The bacterium can be spread among these animals and to humans by the bite of certain species of black-legged ticks. In the Eastern and Central United States, the deer tick spreads Lyme disease while in the Western United States it is spread by the western black-legged tick. Other tick species do not spread Lyme disease, although they may harbor other tick-borne diseases.

Ticks are small insects that feed on the blood of animals. When a tick feeds on an infected animal, it acquires the Borrelia burgdorferi bacterium. Then, if that tick feeds on a person, it transmits the bacterium into the person’s bloodstream through its bite. The tick remains capable of spreading the disease for the rest of its life. While deer are a primary host of ticks, deer themselves do not become infected with the Borrelia burgdorferi bacterium. However, they help to spread the disease by transporting infected ticks.