my uncle is having a pus and kind of a big painful cavity in his shin area of his leg. followed by pus formation and intense pain and unable to walk. and it has almost reached his bone. so it is very very painful and cannot walk. started with a small boil and intensified and increased into a big cavity. He has seen dermatologists but they cannot find out what is he suffering from. They just do cleaning and cover with band aid. please let me know what is he suffering from and what are the steps for prevention?