Lyme disease is a bacterial infection resulting from the bite of an infected tick. The most common type of tick that carries the bacteria is the tiny deer tick, or black–legged tick, which is about the size of a poppy seed. Another tick thought to spread the disease is the lone star tick.

A tick picks up the Lyme disease bacteria, called Borrelia burgdorferi (B. burgdorferi), when it bites an animal that is infected with it. .

When an infected tick attaches to you and maintains contact with your blood, the bacteria can travel from the tick’s gut to your bloodstream. Once the bacteria enter the bloodstream, they can move to different parts of the body. Common sites of infection include the skin, joints, muscles, nerve tissue, and distant skin sites. Ticks are most likely to transfer the infection to you after being in contact with your blood for two or more days.

According to the US Centers for Disease Control and Prevention (CDC), the number of Lyme disease cases per year ranged from 19,804 to 29,959 from 2002-2011. The disease is concentrated in certain parts of the country where both the ticks that carry the Lyme bacteria and the mice, deer, and chipmunks that the ticks live on are common. Although Lyme disease is most frequently associated with the Northeast United States, it has been reported in nearly all states.