Some excellent suggestions there, and confirmation of some of the things I was thinking. I take Echinacia (with a break after marathons), vitamin C, sometimes Zinc and was considering other supplements, some of which have been mentioned.
I'm considering doing more during my final taper week, not high intensity or mileae, but not as little as I have been - I'd discussed the stress hormone thing last night with my Chiropractor and she said it's a bit like going on holiday and getting ill. Your stress levels have been greatly reduced so your body decides it's time to relax and repair itself and in turn is a bit suceptable to bugs.
Final thing which I didn't mention originally is that I don't eat meat (I do eat fish, but not every day). My chiropractor also felt my protein intake isn't high enough particualry during the high mileage phase, and that is kicking in later on in the process, just before the race.
I'm having a bit of a problem with getting hit with a chest infection just before a marathon (happened 5 times now). I tend to follow the same training plan and have a 3 week taper, with the last week consisting of a couple of short runs and perhaps a swim. During that week, for the last 7 marathons I've trained for, I've ended up picking up a chest infection mid week and either running a poor time, or just accepting I'm not going to make it and pulling out. It's all a bit frustrating after spending 4 months training well, only to be prevented from achieving what I'm after by a cold going onto my chest. I do have a history of asthma. After any adice or suggestions, or even a recomendation of which heath care professional I could ask, as I'm completely stumped would be much appreciated. GP was sypathetic, but not a massive amount of help.