Light micrograph section through a human heart muscle showing infective myocarditis. Myocarditis is an inflammation of the heart muscle (myocardium). This can affect the heart muscle and the heart's electrical system, thereby, reducing the heart's ability to pump, causing rapid or abnormal heart rhythms (arrhythmias). A viral infection usually causes myocarditis, but it can result from a reaction to a drug or be a part of a more general inflammatory condition.