I have found large doses of chewable (kids) vitamin C (say 1000mg to 2000mg, an adults dose!) before going out (and after even) and pulling an 'all nighter' in tough cold conditions - Will (for me personally - i am prone a bit) most of the time prevent the occasional cold/flu getting hold as a result of breathing very cold air all night and being a bit run down possibly, and tired. Many, many people, i know, dont beleive in vitamin C's properties and poh poh its claims - thats fine
- doesnt worry me if you dont believe this. It might work for a few people maybe?
And dont forget a little bit of chocolate can be your friend out there!(contains caffiene of course- some mountain climbing/trecking friends swear by it at 5000mtrs)
Cheers
Kearn