The Only Use of

fester
in
Where the Red Fern Grows

Every time I'd see a coon track down in our fields, or along the riverbanks, the old sore would get all festered up and start hurting again.†
p. 19.3

festered = (of an injury) became inflamed and created pus OR (generally) got worse -- especially through inattention

Definition:

to grow worse or more intense over time, especially when ignored; or, in the case of a wound, to become inflamed and filled with pus