As has already been said, AC doesn't cause the common cold; viruses do. I can't get the flu shot for medical reasons, and I've had pneumonia a couple times, which leaves me extra susceptible to respiratory illnesses, so I am very, very careful during cold and flu season. Per doctor's advice, I don't fly and I avoid other places where lots of folks are packed into confined spaces, like Disney buses. (I realize that's easy for me to say - I live near WDW and drive there, anyway.) I wash my hands before eating, and never, ever touch my face/eyes/mouth without first washing my hands. At WDW, I avoid touching things like railings, etc. as much as possible. I keep my distance from anyone coughing or sneezing. In any hotel room, I wipe down light switches, door handles, TV remotes, counters, etc. with Lysol wipes. I get adequate rest and sleep, and I eat healthy foods that promote a good immune system. If I do feel an illness coming on, I take the time to rest and recover instead of pushing through and making things worse. <knock on wood> It's no magic pill, but it's worked for me so far!