pus vs festering

pus and festering both are nouns.

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

As nouns, pus and festering are synonyms defined as:

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

pus (noun)	festering (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