Rheumatic fever is an inflammatory disease that can develop when strep throat or scarlet fever isn't properly treated. Strep throat and scarlet fever are caused by an infection with streptococcus (strep-toe-KOK-us) bacteria. Rheumatic fever most often affects children ages 5 to 15. But it can develop in younger children and adults. Although strep throat is common, rheumatic fever is rare in the United States and other developed countries.