It is a type of Staff infection in their foot. It will look like a black spot on the bottom for the most part, and there might be swelling and a reddish color around the dark spot. I have just encountered my first bumble foot so i'm a little new at this also. You have to remove the black spot and all the puss that is inside. Then you fill it with antibiotic cream and wrap it up. THats about all I know.