Interesting thread. I have begun to wonder about the respiratory issue. Not just colds but serious throat infections and chest infections after long haul travel to Oz over the past 2 years. Two trips per year, total 4 return journeys in Y on different airlines and after each anti-biotics necessary. Ok, maybe it is just me.
Much more interesting was a mysterious illness landing me in hospital after an Air Wales (now defunct) flight. I flew LCY to Swansea. This necessitated a stop in Cardiff. The engines were left running, the rear door was open all the time and there was a nasty exhaust smell in the cabin. We continued to Swansea. Within an hour I had unpleasant tightness and pain across the upper chest and pains in my left arm. I functioned perfectly well all afternoon and evening and it eventually eased off overnight. The next day the return journey via Cardiff and the same procedure again on the ground. By the time I got off at LCY and made my way to the underground I was very unwell indeed with the same symptons. 999 call and London ambulance arrived but they did not think it was a heart attack. Taken to the London Hospital I think it was. Various blood tests and ecg and things and no one could work out what the blip on the ecg was. I was kept in overnight and then subjected to all kinds of heart tests including the treadmill and was running "uphill" at the end of 13 minutes. They said they don't see heart patients doing that. I was discharged. No further symptons. I mentioned this to a chemistry teacher friend who said the exhaust fumes from the aircraft has chemicals in it which can act like heart stimulants used by the medical profession and he was not at all surprised by what I experienced.