[quote=Revans Exile]
If you are immune to a disease you can't be infected ever, it is a physical impossibility.
[/quote]

But you can be immune to one strain, yet not another. In this case, the disease may need to undergo a mutation to become airborne. The abberants can be immune to this airborne strain while still being susceptible to a strain that lives in solution. So the scenario is quite plausible.