1. to form or discharge pus; to become infected or inflamed
Usage: often used of wounds or sores
2. to become worse or more intense, especially when left unaddressed
Usage: figurative use

Examples

• The cut on his hand festers if he doesn't keep it clean.
• Without treatment, the wound festers and becomes painful.
• Resentment festers when people don't communicate openly.
• The unresolved conflict festers beneath the surface of their friendship.
• If you ignore the problem, it will only fester over time.
• The infected sore festers and spreads to nearby skin.