Myocarditis is a condition in which inflammation of the myocardium, the middle muscular layer of the heart wall, causes chest pain, shortness of breath, or rapid or irregular heartbeats. The myocardium, or heart muscle in layman's terms, makes up the majority of the heart. Like other muscles, the heart muscle can become inflamed. This can be caused by bacteria, viruses, fungi, or autoimmune diseases. In myocarditis, which accounts for approximately 25% of heart muscle diseases, heart muscle cells are damaged.