harshly uninviting or formidable in manner or appearance; "a dour, self-sacrificing life"; "a forbidding scowl"; "a grim man loving duty more than humanity"; "undoubtedly the grimmest part of him was his iron claw"- J.M.Barrie
a widespread outbreak of an infectious disease; many people are infected at the same time
(especially of medicine) of disease or anything resembling a disease; attacking or affecting many individuals in a community or a population simultaneously; "an epidemic outbreak of influenza"