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Definition of pestilential : 1. Having the nature or qualities of a pestilence.
2. Hence: Mischievous; noxious; pernicious; morally destructive.

Synonyms:

injurious, detrimental, foul, evil, infectious, pesky, vexatious, dirty, nettlesome, insalubrious, noisome, dangerous, virulent, malignant, teasing, harmful, pernicious, destructive, pestilent, poisonous, irritating, ruinous, corruptive, unhealthful, perverting, noxious, galling, baneful, bothersome, perversive, deleterious, annoying, plaguelike, deadly, plaguey, vexing, unwholesome, plaguy, mischievous, pestiferous, help, hurtful, epidemic, bad, pestering

troublesome

Usage examples:

• He had a curious recollection of how his knowledge of Inchling and his wife being always in concert, entirely- whatever they might think in private- devoted to him in action, had influenced, if it had not originally sprung, his resolve to cast off the pestilential cloak of obscurity shortening his days, and emerge before a world he could illumine to give him back splendid reflections. - "One of Our Conquerors, Complete", George Meredith Last Updated: March 7, 2009.
• On our departure the next morning, he sent us a present of a horse; an indifferent one, 'tis true, but, at least, it marked his kindly feeling; he warned us not to delay longer than was absolutely necessary in the country of Meer Moorad Beg, whom he described in no very flattering terms; and he, moreover, cautioned us against the Koondooz fever, which he declared would inevitably attack us if we were not very careful in selecting our encamping ground at a distance from the pestilential marshes which skirted the bases of the hills. - "A Peep into Toorkisthhan", Rollo Burslem.
• But as the hardest granite rock yields and breaks under the drop of water which incessantly falls upon it, so that great nation had to break and to fall into pieces under, not the drop, but the rivers of impure waters which for centuries have incessantly flowed in upon it from the pestilential fountain of the confessional. - "The Priest, The Woman And The Confessional", Father Chiniquy.