If the skin was broken before the wound healed over, it could have entered there. Also, for some reason, my wife develops infection under bubble blisters that has never been opened. I'm not sure if the infection is present in the skin or blood but infections can develop in internal areas that are not exposed.So inadequate debridement after original burn? but why would sterile acid introduce infection. It would only violate the tissue, and it must’ve got something important like blood supply. Diabetes maybe. Abscesses don’t really get ulcerated they cause ulcer from pressure necrosis of the skin. The sulfuric acid may have caused the initial injury… But I would assume he lost a limb from pre-existing micro vascular disease or incomplete debridement.