Myocarditis can result from a wide variety of infectious organisms, autoimmune disorders, exogenous agents, and genetic and environmental factors. Viral infections are the most common cause of myocarditis in the United States and Western Europe. The more common viruses include coxsackievirus A and B strains and, possibly, poliomyelitis, influenza, rubeola, rubella, and adenoviruses and echoviruses. Bacterial infections that may cause myocarditis include diphtheria, tuberculosis, typhoid fever, tetanus, and staphylococcal, pneumococcal, and gonococcal infections. Helminthic infections, such as trichinosis and parasitic infections, especially trypanosomiasis (Chagas disease) in infants and immunosuppressed adults, can be the cause, as can toxoplasmosis. Other causes include hypersensitive immune reactions such as acute rheumatic fever, systemic immune diseases, postcardiotomy syndrome, radiation therapy, and certain drugs and chemicals. Venemous bites or stings that may result in myocarditis include those of the scorpion, snake, black widow spider, and wasp.

Complications from myocarditis include arrhythmias, thromboembolism, chronic valvulitis (when disease results from rheumatic fever), and recurrence of disease. Occasionally left-sided heart failure can occur, and cardiomyopathy is rare.

Myocarditis usually causes nonspecific symptoms, such as fatigue, dyspnea, palpitations, and fever that reflect the accompanying systemic infection. Occasionally, it may produce mild, continuous pressure or soreness in the chest. The patient history commonly reveals recent febrile upper respiratory tract infection, viral pharyngitis, or tonsillitis. Physical examination shows supraventricular and ventricular arrhythmias, S₃ and S₄ gallops, a faint S₁, possibly a murmur of mitral insufficiency (from papillary muscle dysfunction) and, if pericarditis is present, a pericardial friction rub. Although myocarditis is usually self-limiting, it may induce myofibril degeneration that results in right- and left-sided heart failure, with cardiomegaly, jugular vein distention, dyspnea, persistent fever with resting or exertional tachycardia disproportionate to the degree of fever, and supraventricular and ventricular arrhythmias. Sometimes myocarditis recurs or produces chronic valvulitis (when it results from rheumatic fever), cardiomyopathy, arrhythmias, and thromboembolism.