Strep Throat About 10% of children who get a sore throat and fever are infected by Group A streptococci (strep throat). Strep throat is a sore throat caused by bacteria, not a virus. The bacteria infects the throat and cause pharyngitis, inflammation of the pharynx. Strep throat is important to get treated for quickly because certain kinds of strep may cause serious problems if left untreated. Problems that can come from untreated strep include rheumatic fever, which can cause arthritis and heart problems, or nephritis which is a kidney inflammation resulting in bleeding into the kidney and urine. Scarlet fever, is another, non-serious, but common complication of strep t