(1):
(v. i.) The special contagion, inappreciable to the senses and acting in exceedingly minute quantities, by which a disease is introduced into the organism and maintained there.
(2):
(v. i.) Contagious or poisonous matter, as of specific ulcers, the bite of snakes, etc.; - applied to organic poisons.
(3):
(v. i.) Fig.: Any morbid corrupting quality in intellectual or moral conditions; something that poisons the mind or the soul; as, the virus of obscene books.