Diseases transmitted by the bite of a tick or mite affect both humans and animals. Borrelia burgdorferi is a leading cause of Lyme disease in the United States. The bacterium is transmitted to humans through the bite of infected blacklegged ticks. If left untreated, infection can spread to joints, the heart and the nervous system. Also transmitted from a tick bite, Babesia is a protozoan parasite of the blood that causes a hemolytic disease known as babesiosis. In the United States, Babesia microti is the most common strain associated with humans with other species infecting cattle, livestock and occasionally domestic animals. Scrub typhus, also known as bush typhus, is an acute, febrile disease caused by a bacteria called Orientia tsutsugamushi. Tularemia, a potentially serious illness that occurs naturally in the United States, is caused by the bacterium Francisella tularensis and found in animals and transmitted in a variety of ways, including the bite of an infected tick or insect.