dodss, you might be right about transfers this time. We had to use the automated train between terminals in O'Hare and that felt like we were riding in a giant Petri dish.
You could always do as I do and not fly.
I have had a cold or similar after flights.The subject of coughs and colds after flights was discussed with an aero expert on Rip off Britain a few weeks back. The guy largely debunked the myth that it was the plane air circulation. He said it was not recirculated and certainly not from back to front of the plane to everyone but just circulated in a circle a few seats around you.....
As a former aircraft maintenance technician I have to say it most certainly is recirculated. It goes back into the heating (or air conditioning) system to be reheated or recooled before being blown back into the cabin.
Some of it is recirculated. Some of it is fresh, drawn in via the engines.
Breathing airplane air is really no different than breathing enclosed office air. It is a mixture of fresh and recirculated air....