Rheumatic heart disease starts as a sore throat from a bacterium called Streptococcus pyogenes (group A streptococcus) which can pass easily from person to person in the same way as other upper respiratory tract infections. Strep infections are most common in childhood. In some people, repeated strep infections cause the immune system to react against the tissues of the body including inflaming and scarring the heart valves. This is what is referred to as rheumatic fever. Rheumatic heart disease results then from the inflammation and scarring of heart valves caused by rheumatic fever.