Synonyms for Pernicious (same or very similar meaning)
WordNet sense 1 (exceedingly harmful):
baneful, deadly, pestilentWordNet sense 2 (injurious to physical or mental health):
harmful, noxiousWordNet sense 3 (working or spreading in a hidden and usually injurious way):
insidious, subtleFrom the ODE community, based on WordNetadd/edit