Rocky Mountain spotte fever is the most severe and most frequently reported rickettsial illness in the United States. It has been diagnosed throughout the Americas. Some synonyms for Rocky Mountain spotted fever in other countries include “tick typhus,” “Tobia fever” (Colombia), “São Paulo fever” or “febre maculosa” (Brazil), and “fiebre manchada” (Mexico). It should not be confused with the viral tick-borne infection, Colorado tick fever. The disease is caused by Rickettsia rickettsii, a species of bacterium that is spread to humans by ixodid (hard) ticks. Initial signs and symptoms of the disease include sudden onset of fever, headache, and muscle pain, followed by development of rash. The disease can be difficult to diagnose in the early stages, and without prompt and appropriate treatment it can be fatal.