fester
- For negative feelings to get worse because they are not being dealt with
After their fight, Joan and David had a long talk to patch things up, because they didn't want to let any bad feelings fester.
- For a sore to become inflamed and form pus, usually as a result of neglect
Fred was so busy at work, that he ignored the cut he got while cutting vegetables, and after a week he realized that the wound had begun to fester.