“Strep” is an abbreviation for the bacteria Streptococcus. There are several species, and each causes a different type of disease. When we say “Strep” we tend to mean the species that causes Strep Throat; we refer to the others differently. They come into play as follows:
Group A beta-hemolytic Strep (GABHS), also called Streptococcus pyogenes, causes Strep throat. It also causes rapidly-advancing skin infections like cellulitis; and if it gets inside the body, can cause severe pneumonia and septic shock, or uterine infections after childbirth. Penicillin works well to kill GABHS, unless it has advanced too quickly, producing a unique toxin. Jim Henson, creator of The Muppets and Sesame Street, died of toxic shock from pneumonia due to Strep.
Group B Strep is a different species, and is thus a different germ. It’s present in the vagina of 25% of women, where it lives harmlessly. During pregnancy it can cause urinary tract infections. But it’s most dangerous after childbirth, when it can cause severe uterine infection in the mother, and fatal sepsis in the baby. Penicillin works very well against it.
Groups C and G Strep are uncommon. They can cause the same types of infections as GABHS, but cannot cause Rheumatic Fever, a unique complication of Strep Throat (from GABHS). Group D only causes rare heart and bloodstream infections.
Streptococcus pneumoniae is a completely different germ; we refer to it as “Pneumococcus”. It’s the most common bacteria to cause pneumonia, otitis media (middle ear infection), and sinusitis. It can also cause meningitis, and overwhelming sepsis. Penicillin works well, although strains are gradually becoming resistant to it. We have two vaccines against it, an older cheaper one recommended for people with general risks for pneumonia (smokers, chronic heart or lung disease, diabetics, alcoholics, etc.), and a newer expensive one for people at risk of pneumococcal sepsis (small babies, elderly over 65, people on dialysis, or with diseases of the immune system).
Streptococcus viridens is a bacteria commonly found on the skin. It doesn’t cause disease, unless it gets into the blood stream and causes Endocarditis of a damaged heart valve. We’d only suspect it if we found it in two or more blood culture specimens.