From slide 4: "Cases [of myocarditis] have tended to be clinically mild" I'm not a cardiologist but is there really such thing as a "mild" case of myocarditis?

From slide 4: "Cases [of myocarditis] have tended to be clinically mild" I'm not a cardiologist but is there really such thing as a "mild" case of myocarditis? I'm pretty sure that's not a condition I would ever want to affect my heart.