Tick bites spread Lyme disease.
Lyme disease . The most common sign of infection is an expanding area of redness on the skin , known as erythema migrans , that begins at the site of a tick bite about a week after it has occurred . Usually , the tick must be attached for 36 to 48 hours before the bacteria can spread . Lyme disease is the most common disease spread by ticks in the Northern Hemisphere .