Exceedingly harmful.

synonyms: baneful, deadly, pestilent.

Roget 649: hurtful, harmful, scathful^†, baneful, baleful; injurious, deleterious, detrimental, noxious, pernicious, mischievous, full of mischief, mischief-making, malefic, malignant, nocuous, noisome; prejudicial; disserviceable^†, disadvantageous; wide-wasting.    unlucky, sinister; obnoxious; untoward, disastrous.    oppressive, burdensome, onerous; malign etc. (malevolent) 907.    corrupting etc. (corrupt) etc. 659; virulent, venomous, envenomed, corrosive; poisonous etc. (morbific) 657^†; deadly etc. (killing) 361; destructive etc. (destroying) 162; inauspicious etc. 859.    bad, ill, arrant, as bad as bad can be, dreadful; horrid, horrible; dire; rank, peccant, foul, fulsome; rotten, rotten at the core.    vile, base, villainous; mean etc. (paltry) 643; injured &c.; deteriorated etc. 659; unsatisfactory, exceptionable indifferent; below par etc. (imperfect) 651; illcontrived, ill-conditioned; wretched, sad, grievous, deplorable, lamentable; pitiful, pitiable, woeful etc. (painful) 830.    evil, wrong; depraved etc. 945; shocking; reprehensible etc. (disapprove) 932.    hateful, hateful as a toad; abominable, detestable, execrable, cursed, accursed, confounded; damned, damnable; infernal; diabolic etc. (malevolent) 907.    unadvisable etc. (inexpedient) 647; unprofitable etc. (useless) 645; incompetent etc. (unskillful) 699; irremediable etc. (hopeless) 859.    ... show more

Working or spreading in a hidden and usually injurious way.

synonyms: insidious, subtle.