I think that's unhelpful and alarmist. I know plenty of people who are nearly always unwell after longhaul flights, it's extremely common - although often due to a combination of lack of sleep from travelling and the dry air on the plane which can exacerbate symptoms. Additionally, airports, hotels etc. are breeding grounds for viruses given close proximity of people from multiple different areas - just because someone is ill after a flight they didn't necessarily catch a virus ON the flight. Travel by its nature increases exposure to cold and flu viruses.