I don't think anyone was still sick at that point. They had the antibiotics and Herschel was treating all the sick before he was capture by the Gov. They were weak and recovering, but not under the influence of the virus anymore. Those who died from the virus died in the prison before the you-know-what hit the fan. I remember the big fight when a bunch of the infected turned and Lizzie killed one by luring it away and Herchel almost died fighting one. Then Glen almost died, but they got the medicine and were treated. Even if one did turn in the bus, they could have opened the door and ran out.
I just found it odd that they were all in the bus dead. Like another poster said, if they ran out of gas, they would have scattered in the woods. There were no walkers around the bus, so it's not like they were attacked, the bus was buttoned up tight and everyone was just inside. No crash or accident, no breach, just looked like the bus stopped and they all died. If anyone wanted out, they could have ran out, instead they were all just in the bus. I hope they explain what happened to them.