It’s a foot fungus, and it stinks. Air those piggies, then soak them in salty water, wash them with garlic juice, or soak them with diluted white or apple cider vinegar. All of these things will help kill the fungus. But you have to be persistent, consistent and diligent: no matter what treatment you use, do it a few times a day and stick with it until at least a week after you think the symptoms are gone! Fungus excels at hiding out and coming back when you least expect it.