Streptococcal bacteria are divided into several categories, depending on their ability to break down red blood cells and the composition of their cell walls. Many healthy people harbor streptococcal bacteria in their mouths, noses, intestines, and reproductive tracts; on their skin; and in other locations without becoming sick; these people are called “carriers”. Although people who are ill with streptococcal disease are the most likely sources of many streptococcal infections, some infections may come from carriers of streptococcal bacteria. Streptococcal bacteria can cause a variety of illnesses that range from mild to severe.
Group A streptococcal bacteria cause diseases ranging from streptococcal sore throat (strep throat) to necrotizing fasciitis (flesh-eating disease). They can also cause scarlet fever, rheumatic fever, puerperal (postpartum) fever, and streptococcal toxic shock syndrome. Group A strep infections are most often transmitted by ill persons, although carriers of group A streptococci are the sources of some infections.
Group B streptococci can cause life-threatening sepsis, pneumonia, or meningitis in newborns, as well as a variety of illnesses in the elderly and adults with certain underlying conditions or weakened immune systems. Some people normally carry group B strep in their gastrointestinal tract, genital tract, or urinary tract without showing symptoms. Up to 30% of pregnant women carry group B streptococci in their genital tract and may pass the bacteria to their infants during birth (especially for infants who develop disease in the first week after birth). The source of infection is often unknown for infants with group B strep illness that begins more than a week after birth, and for adults with group B strep disease.
Streptococcus pneumoniae bacteria, or pneumococcus, can cause many types of illnesses, and some of them are life-threatening. Pneumococcus is the most common cause of bloodstream infections, pneumonia, meningitis, and middle ear infections in young children. S. pneumoniae bacteria are often present in the nose or throat of healthy people and only develop into a serious infection when the host's defenses are weakened due to such factors as very young or old age, illness (e.g., heart disease, lung disease, cancer, etc.), or malnutrition. Vaccines are available that provide protection against some types of S. pneumoniae bacteria.