If a cut or other injury festers, it becomes infected and produces pus.

a festering sore 脓疮

If an argument or bad feeling festers, it continues so that feelings of hate or lack of satisfaction increase.

It's better to express your anger than let it fester inside you. 把愤怒表达出来比郁积在心里要好。

a festering argument/dispute 愈益激化的争论／争端