WordNet sense 1 (exceedingly harmful):

baneful, deadly, pernicious

WordNet sense 2 (injurious to physical or mental health):

harmful, noxious

WordNet sense 3 (likely to spread and cause an epidemic disease):

pestiferous, pestilential, plaguey

WordNet sense 4 ((especially of medicine) of disease or anything resembling a disease; attacking or affecting many individuals in a community or a population simultaneously):

epidemic