MYOCARDITIS - Inflammation of the heart muscle layer (myocardium) Most commonly the result of an infection. Can exist in chronic and acute forms. In the U.S. the most common cause for acute myocarditis is viral (Coxsackievirus B). Other causes include: bacterial infection (Lyme disease), HIV viral myocarditis, radiation myocarditis, protozoal (Chagas disease of Central and South America), and giant cell myocarditis (cause unknown).