Hard to say... apart from some flippant suggestons that spring to mind!
Have you been doing long mileages? For example, I presume you know that marathon running is known to lower the immune system.
I'd say that 'sleep well' is more important than eat well... although a good diet is still important.
When I know I'm tired tired / stressed... I do take a bit more care in avoiding risks... e.g. if I shake hands with people, I do try to avoid touching my eyes/mouth for a few minutes.
Edited: 20/03/2013 at 14:53