Rheumatic fever is an inflammatory disease that can develop as a complication of inadequately treated strep throat or scarlet fever. Strep throat and scarlet fever are caused by an infection with streptococcus bacteria. Rheumatic fever is most common in 5- to 15-year-old children, though 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. However, rheumatic fever remains common in many developing nations. Rheumatic fever can cause permanent damage to the heart, including damaged heart valves and heart failure. Treatments can reduce damage from inflammation, lessen pain and other symptoms, and prevent the recurrence of rheumatic fever.