festering vs pus

festering and pus both are nouns.

Word	Noun	Adjective	Verb	Adverb
festering	Yes	No	No	No
pus	Yes	No	No	No

As nouns, festering and pus are synonyms defined as:

Other synonyms of festering include ichor, purulence, sanies, suppuration.

festering (noun)	pus (noun)
a fluid product of inflammation	a fluid product of inflammation
(medicine) the formation of morbific matter in an abscess or a vesicle and the discharge of pus