Autoantibodies. Rheumatic heart disease is one of the complications of an untreated strep infection. A month or more after the infection, the body has made antibodies that mistale the tissue of the heart for the germ they were designed to fight. Over time thease antibodies cause inflamation of the heart and can injure the valves. After the acute phase is past, you have a weaker heart with leaky valves and murmurs.
Immune response stre. It is the body making an immune response to a strep infection. The immune reaction cause antibodies to heart tissue and can lead to the body making a response that causes damage to heart valves causing leaking valves especially the mitral valve.