Rheumatic fever is an autoimmune disease. It most often happens after an infection. Our immune system protects us from diseases and infection. With this illness, your immune system attacks the healthy cells in the body. Rheumatic fever is most often a side effect of strep throat or scarlet fever. It is not passed from person to person, but the strep infection may be. Rheumatic fever affects the heart, joints, skin, and brain. It is more common in children. If not treated the right way, this illness can cause very serious heart problems.