Scarlet fever is a term used for strep throat with a rash. Scarlet fever is most common in children ages 2 to 10, but it can affect people of any age. Scarlet fever is caused by streptococcal (strep) bacteria, the same bacteria that cause strep throat. There are many different strains of strep bacteria, some of which cause more serious illness than others. The type of strep that infects the throat and causes scarlet fever is called group A beta-hemolytic streptococcus (GABS).