How much "more" or "less" likely are you to catch a cold or a "bug" on an airplane than just out in public?

its more likely on a plane, or anywhere air-conditioned, than out in public because the nasal mucosa can get cold and dehydrated making it easier for viruses to colonize the nose. also in enclosed spaces the microbes to circulate and travel between people than outdoors.

flying a lot exposes you to additional radiation which isn't great for you either - that's why air stewardesses have higher rates of breast cancer.

One is slightly more likely to catch a contageous respiratory infection when flying and sitting close to an infected person. The longer aboard, the more likely. The stress of frequent flying would also contribute.

This can't be a major issue or we'd hear more about it.

"There is no evidence that re-circulation of the air in aircraft cabins increases the risk of spreading infections amongst the passengers since very effective filters are used to remove bacteria and viruses."

Closest thing to a fact I could find.