Myocarditis is frequently caused by viral infections, but animal models that closely resemble human disease suggest that virus-triggered autoimmune disease is the most likely cause of myocarditis. Myocarditis is a rare condition that occurs primarily in men under age 50. The incidence of myocarditis rose at least 15x during the COVID-19 pandemic from 1-10 to 150-400 cases/100,000 individuals, with most cases occurring in men under age 50. COVID-19 vaccination was also associated with rare cases of myocarditis primarily in young men under 50 years of age with an incidence as high as 50 cases/100,000 individuals reported for some mRNA vaccines. Sex differences in the immune response to COVID-19 are virtually identical to the mechanisms known to drive sex differences in myocarditis pre-COVID based on clinical studies and animal models. The many similarities between COVID-19 vaccine-associated myocarditis to COVID-19 myocarditis and non-COVID myocarditis suggest common immune mechanisms drive disease.