As a teacher I'm exposed to a LOT of "bugs" throughout the year, and several of us swear by echinacea. It's an herb that historically has been used by Native Americans to stay healthy. It's available at GNC and places like that, but at $12-$15 a bottle. I get mine at Wal-Mart for about $5. In fact, we're heading to WDW tomorrow (yay!), but DS was sick last week, and I started taking the echinacea as a preventative measure. I did get a bit of a head cold, but kept taking the herb and the symptoms were very mild and only lasted a day or two. Of course, it's always a good idea to check with your doctor before taking anything, even an "all-natural" herbal supplement.
Good luck - I hope you recover soon and have a wonderful trip!
P.S. Just noticed you're from Ireland - I hope that echinacea is available to you if you want to try it; otherwise, my commiserations and continued well wishes!