Bumblefoot is a staph infection. There is staph in dirt - so if the bird gets some sort of injury to the foot - a sticker, splinter, sharp rock, etc. - that leaves a small wound, the staph can get in and take hold. There are people who have had some success getting rid of it without surgery - you'll have to research on this forum to find it. Something about soaks in a fish antibiotic?? My two best resources that I learned from are:
If you do not get every staph "kernel" and/or the infection spreads and becomes systemic, you have to use penicillin injections to cure it. It definitely will kill them if it becomes systemic so it's nothing to mess with. Be sure to wear gloves when dealing with it - staph is bad stuff! The surgery is not fun, but you do what you have to do.