Scarlet fever is an infection of the throat caused by group A streptococci bacteria. The disease most commonly starts in children or adults with a 'strep throat' infection A fine red (scarlet) rash develops 12 to 48 hours after symptoms begin. Adult-onset Still's disease causes joint swelling and joint pain. . A sign of scarlet fever is a telltale bright red rash over your body, which typically feels like.