It's a little late but here's what I've been able to gather so far.
If you get a cold even the day before auditions, here's a recipe to follow that will get you through it (at least it works for me).
Make regular hot tea, add a squeeze of lemon, a glob of honey and a pinch of sage. You'll be good to go. Otherwise, drink tons of water to stay hydrated and healthy. Taking vitamins can help ward off sickness, and avoid hot spicy foods for about the last week leading up to auditions (so as not to hurt your throat and prevent rehearsal).
Break a leg.