Rocky Mountain spotted fever is a serious disease that is often fatal. One of the rickettsias, which are germs slightly larger than viruses, causes the disease. The germ infects the Rocky Mountain wood tick and the American dog tick. The ticks become infected when they bite small mammals, such as field mice and dogs, that are infected with the germ. When the tick bites a person, it transfers the rickettsia to that person's bloodstream. Doctors first discovered the fever in the Rocky Mountain area of the United States, but it occurs throughout the country. It is most common in the Southeastern and Middle Atlantic States. About 600 cases are reported every year, usually in late spring or early summer. It begins with chills and fever, and severe pains in the leg muscles and the joints. Then a rash develops. The fever resembles many of the typhus diseases (see Typhus).