http://www.ninds.nih.gov/disorders/lyme/lyme.htm
National Institute of Neurological Disorders and Stroke, National Institutes of Health
Lyme disease is caused by a bacterial organism that is transmitted to humans via the bite of an infected tick. Most people bitten by an infected tick develop a characteristic skin rash around the area of the bite. The rash may feel hot to the touch, and vary in size, shape, and color, but it will often have a "bull's eye" appearance (a red ring with a clear center). However, there are those who will not develop the rash, which makes Lyme disease hard to diagnose because its symptoms and signs mimic those of many other diseases.