virulency vs harmfulness

virulency and harmfulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
virulency	Yes	No	No	No
harmfulness	Yes	No	No	No

As nouns, harmfulness is a hypernym of virulency; that is, harmfulness is a word with a broader meaning than virulency:

virulency: extreme harmfulness (as the capacity of a microorganism to cause disease)
harmfulness: destructiveness that causes harm or injury

Other hypernyms of virulency include injuriousness.

virulency (noun)	harmfulness (noun)
extreme harmfulness (as the capacity of a microorganism to cause disease)	destructiveness that causes harm or injury
extreme hostility	the quality of being noxious