canker

noun

1
a pernicious and malign influence that is hard to get rid of
racism is a pestilence at the heart of the nation
according to him, I was the canker in their midst
2
an ulceration (especially of the lips or lining of the mouth)
3
a fungal disease of woody plants that causes localized damage to the bark

Also useful for “canker”:Scrabble— High-value letters — see what it scores