i normally find i only get a cold if I'm either forced, for some reason, to keep off the bike, or going on annual leave. I think cyclists in general get less colds and flu. I personally think this is bcoz our germs are trapped in snot and end up in the road (i know you do it!). The natural movement of the human body gets rid of crap off the chest (this is why paralysed people get chest infections a lot coz they don't get rid of any and so it stands to reason that cyclists produce a lot of stuff that's get ejected into the road).
having used First Defence a couple of times i can honestly say i think it does work. And this is the second time this week I've heard of using zinc.
it also stands to reason that colds are psychological bcoz they only happen to active cyclists when they are due annual leave, which is why I've been up all night and will be going to the health shop later. Injury forced me off the bike last winter which resulted in a cold which turned into 2 consecutive chest infections followed by bronchitis, the worst month of my life in recent memory. I don't need that on the first proper block booking of annual leave I've had in 3 years.