Poor wound healing is usually due to many causes. You are right to say that infection is one of the potential causes. There are many types of causes of infection ranging from bacterial, viral, fungal, and even parasitic causes. However more common would be due to repetitive trauma to the area , in this case to your foot. Proper foot wear, avoiding heels in the meantime, in-soles, proper hygiene to the area may all help. Other causes include poor circulation, diabetes, underlying bony growth.
The big "c" word - cancer, is something to consider but one shouldn't jump to that conclusion immediately. Common things are common as the saying goes.
I would advise you to see a doctor and perhaps even get a sample of the wound and/or blood test to send it to a lab to have it analyse. This usually tells us if its one of the causes above.
