If untreated and severe it can kill them (go up the leg).
But if mild I have just left it alone (no redness or swelling- just a tiny black mark on the bottom of the foot- I felt it was more dangerous to open it up and do the bumblefoot surgery). I don't know how it would have worked out permanently since I got rid of that chicken though.
Apparently there is a cheesy substance inside (pus) that must be scraped out...then people pack it (look up bumblefoot surgery on BYC) and keep the chicken in a crate for awhile. It is very involved to clean it out.
I hope this helps...something I am not experienced with (surgery) but wanted you to have the above link.
Bumblefoot is a bacterial infection of a cut or abrasion on the bottom of the foot. The bacteria may be staphylococcus, or one of several other possibilities. The infection might resolve on its own, or it might become a large, severe, painful lesion. The bacteria could eventually get into the bloodstream and cause severe illness and death. So the upshot is to treat it as soon as you notice it, even if mild. The milder it is, the more likely you will be able to treat successfully. Some cases might resolve spontaneously, but I would not risk it.
I've had 3 cases and successfully treated all of them non-invasively, as soon as I noticed them, while still mild-moderate. For mild-moderate cases, soaking is often helpful. Some report success with epsom salts. I soaked with an antibiotic known to be effective against staphylococcus, as I describe here: http://ouroneacrefarm.com/bumblefoot-treatment-tricideneo/
Surgery may be the only treatment option in more severe cases. Basically, if it's so bad that a hardened kernel has formed within the swelling, the kernel needs to be removed surgically. Soaking won't make that go away. But don't take surgery lightly. Some people report failure with surgery, requiring multiple attempts and a lot of pain for the bird.