Select the Meaning of the Underlined Word in the Sentence

Select the meaning of the underlined word in the sentence.
A lot of people don′t know you can′t fight a virus with antibiotics.

A) inflammation of the vermiform appendix
B) microscopic infectious agent capable of replicating only in living cells, usually causing infectious disease
C) a very common, mild viral infection of the nose and throat
D) any of a group of tiny infectious agents composed mainly or entirely of protein