godan verb (-mu), intransitive verb
fester; form pus; suppurate
Intransitive verb describing a wound or infection producing pus. Often used in medical or everyday contexts.
傷口が膿んで痛い。
The wound is festering and painful.
消毒しないと膿むことがある。
If you don't disinfect it, it may fester.
膿 is the noun 'pus', while 膿む is the verb 'to fester' or 'to form pus'.
The kanji 膿 is a phono-semantic compound with 月 (flesh) and 農 (nō), indicating a condition of the flesh. The verb うむ is a native Japanese word meaning 'to produce' or 'to bear', here specialized to 'produce pus'.