Lyme disease is an infection caused by the bacteria Borrelia burgdorferi and Borrelia mayonii, which are carried primarily by the deer tick, also known as the black-legged tick. People bitten by an infected tick may develop an inflammatory condition, which first affects the skin and then may spread to the joints, nervous system, and other body systems.
The ticks that cause Lyme disease are tiny, about the size of the head of a pin or a speck of dirt. They can be found anywhere on the body but tend to attach themselves to areas such as the scalp and groin. People who have exposure to ticks but have not been bitten will not be infected by Borrelia, and many who are bitten will not develop Lyme disease. This is because not every tick is infected and because it can take the tick from 24 to 72 hours after attachment to transmit the bacteria.
Lyme disease is found throughout the northern hemisphere, but the strains of bacteria that cause it and the insects that carry it vary from region to region. In the U.S., Lyme disease is the most commonly reported illness spread by insects (vectorborne). It was the fifth most common Nationally Notifiable disease in 2013, but this does not apply across all states. Lyme disease occurs most frequently in northeastern and midwestern states. The vast majority of the cases occur in the spring and summer when people spend more time outside and the ticks are active.