What is a Virus?
A virus is a small infectious agent that can only replicate inside the cells of another organism. The word is from the Latin ''virus'' referring to poison and other noxious substances, first used in English in 1392. ''Virulent'', from Latin ''virulentus'' (poisonous), dates to 1400. A meaning of "agent that causes infectious disease" is first recorded in 1728, The term ''virion'' is also used to refer to a single infective viral particle. The plural is "viruses".