Scarlet fever: A skin condition that is due to a streptococcal sore throat or any other streptococcal infection. The group A streptococcal bacteria produce a toxin that causes a scarlet rash that initially appears on the neck and chest, then spreads over the body, and lasts around 3 days. As the rash fades, desquamation (peeling) may occur around the fingertips, toes, and groin area. Scarlet fever is usually not a serious illness when treated promptly with antibiotics such as penicillin. However, untreated streptococcal infection can cause kidney inflammation (glomerulonephritis) and rheumatic fever. Also known as scarlatina.