I of course am not a surgeon, but GI abscesses often leave fistulas, and they're just evil and difficult to eradicate. Doesn't mean some can't be treated successfully, but they're often involved and complicated.
I constantly yell at my colorectal surgeons that we can go into a woman's uterus and micro-surgically repair a hole in a baby heart, but we can't fix GI fistulas with much more than medical treatments created thousands of years ago?! Madness!!