if its bacterial it can kill them. It might get better on its own, but that is a huge risk you are taking (i don't think its likely for it to get better on its own if its bacterial). Fortunately it most likely isn't going to kill them as quickly as ich, brook, or velvet would, but you definitely want to treat as quickly as possible if you know its not just bullying.
Consider getting a QT and treat all fish next time, not doing so is a huge risk on your investment and also will most likely cause more time/money.
How big is your tank, what is your livestock, what are your parameters?