Lyme disease is a bacterial illness caused by a bacterium called a spirochete. Lyme disease is spread by these ticks when they bite the skin, which permits the bacterium to infect the body.But most people who have a tick bite do not get Lyme disease. Lyme disease is not contagious from an affected person to someone else. Lyme disease can cause abnormalities in the skin, joints, heart, and nervous system.Lyme disease is common in the United States.