caustic vs virulent

caustic is a noun but virulent is not a noun.

Word	Noun	Adjective	Verb	Adverb
caustic	Yes	Yes	No	No
virulent	No	Yes	No	No

As adjectives, caustic and virulent are synonyms defined as:

Other synonyms of caustic include acerb, acerbic, acid, acrid, bitter, blistering, sulfurous, sulphurous, vitriolic.

caustic (noun)	virulent (noun)
the envelope of light rays reflected or refracted by a curved surface or object
any chemical substance that burns or destroys living tissue

caustic (adjective)	virulent (adjective)
of a substance, especially a strong acid; capable of destroying or eating away by chemical action	infectious; having the ability to cause disease
harsh or corrosive in tone	extremely poisonous or injurious; producing venom
	harsh or corrosive in tone