Cardiotropic organisms, mostly viral, are blood borne.
Inflammatory response is to the organisms and necrotic muscle.
Postinfectious immunogenic cases may be due to complexing of antigenic products of organisms with tissues, or cross reacting determinants common to both organism and host tissues. Epidemiology varies with cause. Heart may only be dilated and flabby with no obvious myocardial visible changes. Dense interstitial inflammatory response may be seen as a gray diffuse or focal discoloration of the myocardium. Valves not affected.