Rocky Mountain spotted fever is the most severe and most frequently reported rickettsial illness in the United States, and has been diagnosed throughout the Americas. Some synonyms for Rocky Mountain spotted fever in other countries include "tick typhus", "Tobia fever" (Columbia), "Sao Paulo fever" and "febre maculosa" (Brazil), and "fiebre manchada" (Mexico). The disease is caused by Rickettsia rickettsii, a species of bacteria that is spread to humans by 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. Appropriate antibiotic treatment is initiated immediately when there is a suspicion of Rocky Mountain spotted fever on the basis of clinical and epidemiological findings.