Lyme disease is an infection caused by the bacteria Borrelia burgdorferi and Borrelia mayonii. Lyme disease tests measure Borrelia antibodies in the blood, or in the cerebrospinal fluid (CSF) if there are signs and symptoms of central nervous system disease.
Lyme disease infection causes symptoms that may include a characteristic erythema migrans (EM) or "bulls-eye" rash that spreads from the site of the bite, fever, chills, headache, and fatigue. If left untreated, Lyme disease may progress to cause intermittent joint pain and swelling, meningitis, facial paralysis (Bell's palsy), weakness and numbness in the arms and legs, memory problems, and may rarely affect the heart or eyes. For more on Lyme disease, read the condition article: Lyme Disease.
Image credit: CDC, James Gathany. Image of a Lyme rash.