Caused by the Borrelia burgdorferi bacterium, Lyme disease is transmitted by the bite of certain species of ticks. The bacterium, normally found in mice, squirrels and other small mammals, does not harm these animals—or the deer the ticks feed on—but causes potentially serious disease when transmitted to people. In the Northeast United States, the Lyme disease vector is the black-legged or deer tick (Ixodes scapularis); in Pacific coastal United States, the western black-legged tick (Ixodes pacificus) transmits the disease. Most cases occur in the early spring and summer when the ticks are in the nymph stage, and people are most likely to be outdoors. Nymph-stage ticks are difficult to see with the naked eye.