The disease commonly transmitted by black-legged ticks in eastern North America is Lyme disease.
Lyme disease is a bacterial infection caused mainly by Borrelia burgdorferi in North America. The principal eastern vector is the black-legged tick, Ixodes scapularis, also widely called the deer tick. These ticks occur across much of the northeastern, mid-Atlantic, and north-central United States, as well as parts of eastern Canada.
Ticks usually acquire the bacteria while feeding on infected small mammals, especially rodents. Nymphs are particularly important to human transmission because they are tiny—roughly poppy-seed sized—and often go unnoticed. Adult females can also transmit the infection. Deer help maintain tick populations and transport ticks, but they do not infect ticks with Lyme bacteria.
Lyme disease is often confused with other tick-borne illnesses such as anaplasmosis, babesiosis, and Powassan virus disease, all of which can involve black-legged ticks. Another common misconception is that every tick bite causes Lyme disease; transmission requires an infected tick, and prompt removal lowers risk. Early symptoms can include fever, fatigue, headache, and erythema migrans, the characteristic expanding rash.