Lyme disease is caused by the bacterium Borrelia burgdorferi. The bacterium normally lives in mice, squirrels, and other small animals. It is transmitted among these animals and to humans through the bites of certain species of ticks, most commonly the deer tick. As many as 50 percent of deer ticks may carry Borrelia burgdorferi in the northeastern and north-central United States, where Lyme disease is common.
Deer ticks live for two years and have three feeding stages: larvae, nymph, and adult. When a young tick feeds on the blood of an animal infected with Borrelia burgdorferi, the tick takes the bacterium into its body. The next time the tick feeds, it can transmit the bacterium to a healthy host. Usually the new host is a small rodent, but sometimes the new host is a human. When a tick infected with Borrelia burgdorferi feeds on a human, the bacterium enters the skin through the tick bite and eventually makes its way into the bloodstream. Usually, a deer tick must feed on the blood between 36 and 48 hours before bacteria can be transmitted. An attached tick that has a swollen appearance may indicate that enough time has elapsed to transmit bacteria. Removing the tick as soon as possible may prevent infection.
Most cases of Lyme disease occur in the late spring and summer when the young ticks are most active and human outdoor activity is greatest.