I think you're confusing RE with The Walking Dead
Afaik, the T-Virus isn't airborne.
Of course it's airborne. That's how it got out in the first place.
All the way back in the first movie when that guy was trying to steal a sample of it he broke a vial of T-virus in it's containment lab. The liquid sample evaporated and got into the ventilation system, which is why the Red Queen tried to shut down The Hive. She killed everybody in the lab because she knew they were contaminated and therefore dead anyway, even though none of them, at the time, had been bitten by a T-carrier.