Firstly, identify the pathogen causing it. In many cases it's Pasteurella, but can also be caused by a number of other bacterial or viral agents. Some are easily treatable, some not. Select for good resistance to Pasteurella; most rabbits are/have been/will be exposed to it at some point in their lives but may not go on to exhibit any symptoms. Where symptoms occur it can be treated with antibiotics but the rabbit is likely to relapse in future.