- baneful, deadly, pernicious, pestilent - exceedingly harmful
Antonyms: innocuous, harmless (indirect, via noxious)
- pestilent, pestilential, pestiferous, plaguey - likely to spread and cause an epidemic disease; "a pestilential malignancy in the air"- Jonathan Swift; "plaguey fevers"
Antonyms: endemic, endemical (indirect, via epidemic, ecdemic)
Antonym: ecdemic (indirect, via endemic, epidemic)