Streptococci are spherical, Gram positive bacteria. Commonly they are referred to as strep bacteria. Streptococci are normal residents on the skin and mucous surfaces on or inside humans. However, when strep bacteria normally found on the skin or in the intestines, mouth, nose, reproductive tract, or urinary tract invade other parts of the body--via a cut or abrasion--and contaminate blood or tissue, infection can be the result.

Numerous strains of strep bacteria have been identified. Those streptococci from groups A, B, C, D, and G are most likely to cause disease. While some of these infections do not produce symptoms, and the infected person can become a carrier of the disease-causing bacteria, other strep infections can be fatal.

Primary strep infections invade healthy tissue, and most often affect the throat. Secondary strep infections invade tissue already weakened by injury or illness. They...