Webster's English Dictionary

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virus

vi.rus \'vi--r*s\ n [L, slimy liquid, poison, stench; akin to OE wa-se 
   marsh, Gk ios] poison, Skt vissub-dot>a; in senses 2 & 4, fr. NL, fr. L 
   archaic  1: VENOM  2a: the causative agent of an infectious disease  2b: 
   FILTERABLE VIRUS; specif : any of a large group of submic roscopic 
   infective agents that are held by some to be living organisms and by others 
   to be complex protein molecules containing nucleic acids and comparable to 
   genes, that are capable of growth and multiplication only in living cells, 
   and that cause various important diseases in man, lower animals, or plants 
   2c: a disease caused by a filterable virus  3: something that poisons the 
   mind or soul  4: an antigenic but not infective material (as vaccine lymph) 
   obtainable fr om a case of an infectious disease