Lyme disease is a bacterial infection caused by Borrelia burgdorferi and, in some cases, Borrelia mayonii. It is transmitted to humans through the bite of infected black-legged ticks, also known as deer ticks. If left untreated, Lyme disease can lead to severe complications affecting the joints, nervous system, and heart. However, with early diagnosis and proper treatment, recovery is usually successful.   Causes and Transmission Lyme disease is primarily spread through the bite of infected black-legged ticks (Ixodes scapularis in the U.S. and Ixodes pacificus on the West Coast). The bacteria enter the bloodstream when a tick remains attached for 36 to 48 hours.