Lyme disease, or borreliosis, is an emerging infectious disease caused by bacteria from the genus Borrelia. The vector of infection is typically the bite of an infected black-legged or deer tick, but other carriers (including other ticks in the genus Ixodes) have been implicated. Borrelia burgdorferi is the predominant cause of Lyme disease in the U.S. and Borrelia afzelii and Borrelia garinii are in Europe.

The disease presentation varies widely, and may include a rash and flu-like symptoms in its initial stage, then musculoskeletal, arthritic, neurologic, psychiatric and cardiac manifestations. In a majority of cases, symptoms can be eliminated with antibiotics, especially if treatment begins early in the course of illness. Late or inadequate treatment often leads to "late stage" Lyme disease that is disabling and difficult to treat. Controversy over diagnosis, testing and treatment has led to two different standards of care.