Infections by Streptococci are separated into several categories, depending on the composition of their cell walls. Groups A and B are the most common and devastating human pathogens. Group A Streptococcus bacteria causes disease ranging from streptococcal sore throat (strep throat) to necrotizing fasciitis (flesh-eating disease). They can also cause scarlet fever, rheumatic fever, postpartum fever, and streptococcal toxic shock syndrome.
Group B streptococci cause life-threatening diseases in newborns, pregnant women, the elderly, and adults with compromised immune systems. Group B strep infections are different from other strep infections, in that the individual can be colonized by the bacteria before any symptoms are obvious. This means that people can carry the bacteria in their bodies but are not infected, and do not show any symptoms. Group B strep can be carried in the gastrointestinal tract, genital tract, or urinary tract, and only become dangerous when they invade the bloodstream.
Among group B infections is pneumonia. Pneumonia can be caused by a barrage of different things including viruses and fungi, but is most commonly caused by Streptococcus pneumoniae also called pneumococcus, which is the only type of pneumonia for which there is a vaccine. S. pneumoniae are often present in healthy throats and only develop into a serious infection when the host's defenses are depleted due to such factors as old age, illness (i.e. AIDS), or malnutrition.