Streptococcus is a genus of coccus (spherical) Gram-positive bacteria belonging to the phylum Firmicutesand the order Lactobacillales (lactic acid bacteria).

Most streptococci are completely harmless and not even capable of causing disease in humans, but a handful of strains can be pathogenic, such as those which cause strep throat. In addition to streptococcal pharyngitis (strep throat), certain Streptococcus species are responsible for many cases of pink eye, meningitis, bacterial pneumonia, endocarditis, erysipelas, and necrotizing fasciitis (the 'flesh-eating' bacterial infections). However, many streptococcal species are not pathogenic, and form part of the commensal human microbiota of the mouth, skin, intestine, and upper respiratory tract.  About a third of women have Streptococcus agalactiae (also known as Group B streptococcus or GBS) in their vaginal microbiomes. Although this strain is largely harmless to adults, it can cause serious infections in newborns who acquire it during birth. For this reason, third-trimester screening for GBS is a standard component of pre-natal care, moms who test GBS positive are given a prophylactic antibiotic right before delivery, and the babies are born free of infection.