A genus of Gram-positive cocci that includes several species of bacteria that cause infections in people.

S. pneumoniae and S. pyogenes are the most clinically relevant streptococci. S. pneumoniae, or pneumococcus, is a commensal bacterium that can become invasive and pathogenic. It is a major cause of community-acquired pneumonia and meningitis but can cause a wide range of other infections.

Group A streptococci (GAS, primarily S. pyogenes) typically infect the throat and skin and can develop into serious invasive infections. Group B streptococci (GBS, S. agalactiae) are the leading cause of disease in new-born babies worldwide and show increasing levels of resistance to second line antibiotics such as macrolides and clindamycin.