Streptococcal (strep) infections are communicable diseases that develop when Streptococcus bacteria normally found on the skin or in the intestines, mouth, nose, reproductive tract, or urinary tract invade other parts of the body and contaminate blood or tissue. Some strep infections don't produce symptoms, and some are fatal.

Most people have had some form of strep bacteria in their body at some point in their lives; people who host bacteria without showing signs of infection are considered to be carriers.

Primary strep infections invade healthy tissue (usually the throat). Secondary strep infections invade tissue already weakened by injury or illness, such as the bones, ears, eyes, joints, or intestines. Both primary and secondary strep infections can travel from affected tissues to the lymph glands, where they enter the bloodstream and spread throughout the body.

Numerous strains of strep bacteria have been identified. Types A, B, C...