It's interesting what you all say regarding colds and "forgetfulness" ,I work in a Transport Office on the nightshift 17:00 to 05:00 4on 4off ( not in an aviation industry I should add).
During my shifts I see upward of 40 different people who are in and out of the Office most of the time.
Generally I seem to suffer from the same almost permanent cold/cough etc that you have described previously.
As to the memory don't even go there !
This leads to me to wonder is it the unsocial hours that drags your immmune level down rather than the altitude ?
Or is it the contact in a confined space with other people ?
Gavin