A rare condition in which the heart muscle becomes thick and inflamed and may also become weak. Myocarditis is usually caused by a viral infection, but it may also be caused by bacterial, parasitic, or fungal infections; autoimmune disorders; or being exposed to radiation or certain chemicals or drugs. Signs and symptoms include chest pain, fast or abnormal heartbeat, trouble breathing, fatigue, and swelling of the legs. Other signs and symptoms include fever, headache, body aches, sore throat, and diarrhea. Some patients have mild symptoms or no symptoms at all. Sometimes the symptoms may be severe or life threatening and may lead to heart failure, stroke, or heart attack.