Lyme disease is an illness caused by the bacterium Borrelia burgdorferi. This bacteria is carried by the black-legged tick, also known as the deer tick. When an infected tick bites you, it can transfer bacteria under the skin, leading to infection. If diagnosed and treated early, many patients recover. However, if not diagnosed or treated early, Lyme disease can develop into a chronic illness that leads to long-term, serious health problems and reduced quality of life. In rare cases, Lyme disease can result in death.