Streptococci are small gram-positive bacteria, spherical to ovoid in shape and linked together in pairs of chains. Several species occur as part of normal human flora in the respiratory, GI, and genitourinary tracts. Although researchers have identified 21 species of streptococci, three classes—groups A, B, and D—cause most of the infections. Organisms belonging to groups A and B beta-hemolytic streptococci are associated with a characteristic pattern of human infections. Most disorders due to group D streptococcus are caused by Enterococcus faecalis (formerly called Streptococcus faecalis) or Streptococcus bovis.

Clinically, there are three states of streptococcal infection: carrier, acute, and delayed nonsuppurative complications. In the carrier state, the patient is infected with a disease-causing species of streptococci without evidence of infection. In the acute form, streptococci invade the tissues and cause physical symptoms. In the delayed nonsuppurative complications state, specific complications associated with streptococcal infection occur. These include the inflammatory state of acute rheumatic fever, chorea, and glomerulonephritis. If complications occur, they usually appear about 2 weeks after the acute illness, but they may be evident after a nonsymptomatic illness.

For information on the most common types of streptococcal infections, including diagnosis, treatment, and special considerations, see Comparing streptococcal infections, pages 818 to 825.