Streptococcus: A group of bacteria that causes a multitude of diseases. Under a microscope, streptococcus bacteria look like a twisted bunch of round berries. Illnesses caused by streptococcus include strep throat, strep pneumonia, scarlet fever, rheumatic fever (and rheumatic heart valve damage), glomerulonephritis, the skin disorder erysipelas, and PANDAS. Familiarly known as strep.
