I am surmised your not Ill more really.
Air travel puts you in an incubator on Long haul flights,adds lots of folks from all over the world with their disease process at various stages ( some viruses and bacterium you have never seen before ever and your immune system is going- What?).
Add air that’s through an engine( except on the wonderful Dreamliner and perhaps some newer designs)
A dash of stress, and odd food at odd times.
Sprinkle liberally with time zone disruption to sleep.
Voilà - the lurgy as it’s called in Scotland.
I am sure your vaccinations are up to date, but not only time zones are a side effect of geography- epididimologicaly you are visiting various places where seasonal flu, colds, mass migration patterns of holidaymakers( all of China has just moved round the country,continent and world and its cold there now- colds and flu) all move illness vast distances and a lot of it in aircraft.
Keep that the immune system good,sleep,eat well and hydrate.
I used to consult at various remote clinics in Eastern Europe and Asia and all that travel was challenging but like I suspect lucky I had an immune system eventually that was Popeyes on concentrated spinach. 30 plus Years in various clinical roles also helped develop it( as a trainee doc or nurse you are sick the first year,better second and pretty strong the third.)
Just coming out of a wave of flu in Singapore now- we get at least two here a year- the northern and southern waves- the joy of being at the Crossroads.