Rocky Mountain spotted fever (RMSF) is a severe infectious disease that affects your blood vessels. It is potentially fatal.

Ticks in North, Central, and South America spread the disease.

Specific bacteria cause RMSF. The American dog tick and the Rocky Mountain wood tick carry these bacteria. It passes to humans when an infected tick bites the skin. The bacteria can then pass into the bloodstream.

The bacteria sit in the lining of small blood vessels and multiply. The growth of the bacteria causes irritation and swelling in the blood vessels. Blood and other fluids can then leak out of the blood vessels into the surrounding tissue.