The Lyme disease bacterium (Borrelia burgdorfen) is carried by a group of closely related species of ticks known as Ixodes. Ticks in this group include deer ticks, western black-legged ticks, and black-legged ticks. These tiny terrors are small—typically no larger than a poppy seed—and transmit the bacteria when feeding on warm-blooded hosts, including mice, deer, dogs, and humans. The bacteria enter the skin through the bite during feeding and eventually make their way into the bloodstream.