I’m sure the vocalists will have much to add regarding avoiding illness as it affects them the most.
My son is insistent on keeping a good sleep schedule to stay healthy. Yes, I know this is challenging with the busy audition prep and travel. As well as fitting in a little time to work out. Even if that is just a short run.
For travel itself, he will mask.
And he keeps allergy and cold medicine with him when he travels. The worst part of a cold for brass and wind players is the pressure from congestion. Be sure and try out whichever over the counter medication you will use. Some have side effects which increase nervousness that may be worse than the cold symptoms.