How is it that ticks make you ill? There are several diseases that humans can contract from a tick bite, but the most common and dreaded is Lyme disease. This illness is caused by the bacterium Borrelia burgdorferi and is transmitted to humans through the bite of infected blacklegged ticks. Symptoms can include fever, headache, fatigue, and a characteristic "Bull's Eye" skin rash called erythema migrans. If left untreated, a Lyme disease infection can spread to joints, the heart, and the nervous system. Even with treatment, this is an illness that can have lifelong debilitating effects such as arthritis, fatigue, and even neurological deficits.