Bumblefoot is a staph infection. Stage 1 could just as easily be a stone-bruise, but by Stage 2-5 they will need medical intervention, from a veterinarian or at the very least, medical attention administered by a human (you). Chicken pus is not liquid like human pus, it's thick, cheezy. Once you get into this sort of infection, you'll need to step in to ease their suffering and cure the disease.