Lyme disease is an infection caused by bacteria called Borrelia burgdorferi. These bacteria are transmitted through the bites of ticks, primarily the deer tick. Not everyone who develops symptoms of Lyme disease remembers getting bitten by a tick because the deer tick is very small and its bite can go unnoticed.
Lyme disease is most common in the northeastern and upper mid-western United States. More than 90% of cases have been reported in nine states: Connecticut, Maryland, Massachusetts, Minnesota, New Jersey, New York, Pennsylvania, Rhode Island and Wisconsin. Even within states, there are regions of high risk and others with very low rates of disease. This variation relates to where ticks that carry the bacteria live, breed and come into contact with humans.
Late spring and early summer are the peak times of the year for Lyme disease to be diagnosed. However, the condition is not always diagnosed right away. So, cases still are identified all year long.
Deer ticks can carry other germs, such as Anaplasma, Babesia, and Borrelia miyamotoi. It's not uncommon for people to have one of these infections in addition to Lyme disease.
The usual first symptom is a rash called erythema migrans (EM), which is usually a flat, reddish rash that spreads from the site of the tick bite. The rash usually is larger than 2 inches wide and can grow larger. It often develops a central clear area known as a bull's eye. The rash usually doesn't itch or hurt. Other symptoms at this stage can include fever, muscle and joint aches, fatigue, headache and a severe stiff neck. In some cases, there are two or more of these well-defined rashes.