ocky Mountain spotted fever (RMSF) is both the most serious and the most ommonly reported rickettsial infection in the USA. The causative organism s Rickettsia rickettsii, which is a member of the spotted fever group. R. ickettsii are small, aerobic, obligate intracellular, Gram-negative coccobailli. The disease name is derived from its origins in the Rocky Mountains. nitially known as “black measles,” RMSF was first recognized in the Snake iver Valley of Idaho and the Bitterroot Valley of western Montana in the ate 1890s. In 1906, a medical team led by Howard Taylor Ricketts etermined the role of ticks in disease transmission. Today, most cases in the SA occur in the mid-Atlantic and southern states (Fig 1). RMSF also has een found in Canada and in Central and South America. RMSF is a systemic mall-vessel vasculitis. Clinical presentations range from benign to life hreatening. Early recognition and prompt treatment are keys to reduce ortality from this intriguing illness.