Myocarditis is an inflammation of the myocardium that frequently follows microbial infections. Bacteria, fungi, protozoa, and viruses can cause this disease. Enterovirus and adenoviruses are the most frequent viral causes of myocarditis, although virtually any virus can be an etiologic agent. Multiple pathogenic mechanisms of tissue injury are possible during myocarditis, including injury resulting from microbial infection and from host defense responses to the infection. This article concentrates on the host defense mechanisms and how specific host defense mechanisms against the infection might lead to autoimmunity to cardiac antigens.