Quinn Dombrowski
fester (verb) – to rot or go bad
fester (noun) – a sore (a wound that forms pus or doesn’t heal)

Here are some examples of how you can use it:
The cat gave me a rash which developed into a festering wound.
The parents never asked the children to make peace, so their bad feelings continued to fester.
It's time we cleaned our garden or those weeds won't stop festering.
He didn't clean his cut well and it has become a fester that leaks (= drips) yellow pus.