Lyme disease is caused by bacteria and is transmitted to humans through the bite of infected ticks: By the western black-legged ticks (Borrelia burgdorferi) in the Western, Northeastern, mid-Atlantic, and in North-Central United States, the disease is spread by deer ticks, or Ixodes scapularis. The smaller, immature tick nymphs usually spread the disease because they are so small and difficult to see. In most cases, the tick must be attached for thirty-six to forty-eight hours or more before the Lyme disease bacterium can be transmitted. If infected, a person can experience flulike symptoms, such as fever, headache, and fatigue; for about 80 percent of the cases, a “bull’s-eye” skin rash (called erythema migrans) can be seen around the bite area. The typical treatment is a few weeks of antibiotics, but if it is not treated, the infection can eventually affect joints, the heart, and the nervous system.