Infectious myocarditis is a common condition which often passes unrecognized, and the true incidence is thus unknown. Lymphocytic myocarditis has been recorded in 1.06% of 12,747 unselected routine autopsies performed over a 10-year period. Dilated cardiomyopathy (DCM) has an estimated frequency of 7.5-10% per 100,000 inhabitants per year. Overall, the enteroviruses, and particularly the Coxsackie-B viruses, predominate among viruses as the cause of myocarditis. As new molecular biological techniques have become available, the cytomegaloviruses (CMV) seem to have emerged as a more common cause of myocarditis than was previously recognized. Among the bacterial myocarditides, diphtheric myocarditis has become a serious threat in Russia and adjacent states during the 1990s. Among newly identified bacteria, Borrelia burgdorferi infection is accompanied by cardiac involvement in 1-8% of cases, where myocarditis with conduction disturbances is the most prominent feature. Chlamydia pneumoniae may be associated with myocarditis and sudden unexpected death. In AIDS, myocarditis with variable aetiology occurs in up to 50% of patients, although asymptomatic in most cases. In lymphocytic myocarditis and DCM, enteroviral-specific nucleotide sequences have been detected in about 30% of patients, and CMV-specific nucleotide sequences in 14%. Borrelia burgdorferi may occasionally be implicated in DCM. In this contribution we focus also on sudden unexpected death (SUD) in young athletes, since, in Sweden, an increased frequency of SUD has recently been observed in young orienteers and myocarditis was a common feature.