There is nothing worse than planning a wonderful trip and then feeling ill when you travel. While I don't have all the answers, I do have some common sense ideas that can help you stay healthy starting with the tried and true answer to most everything: WASH YOUR HANDS! Often!
I carry Purell with me always. Even when we are not traveling. It's no fun being sick at home either. We try to remember to use the hand sanitizer when we've had to shake a lot of hands, use a banister or hand rail in the mall, or just open doors in a public place. We're not fanatics and mostly use it before we eat. Again, keeping those hands clean is more than half the battle against what's lurking out there.
Remember to get your immunizations. The flu shot especially but there are also lots of other immunizations for pneumonia, shingles, etc. that would be good to have. Visit a health clinic if you are planning a trip to a country that you're not sure will have some other problems that could affect your health like malaria.
Don't drink the local water. We follow this rule even when we travel within the USA. Water is different in every area of the country/world and certainly not always processed well. The best advice we got when we went to China was to be sure we ate only cooked food because raw vegetables might have been washed in local water that has poor quality. We were also cautioned to be sure our bottled water was sealed and if kept cold in ice, it was suggested we wipe the bottle off before drinking from it. Remember if you get ice in a drink, it is probably made from local water.
Hydrate, hydrate, hydrate. The best thing you can do for yourself is to drink plenty of water starting from the time you get on the plane to when you return home. Dehydration in any type of climate is possible and you will feel much more energy and wellness if you remember to drink plenty of water.
Stick to a routine as much as possible. Certainly time zone changes are a challenge but once you adjust a bit, keep to a morning wakeup and regular bedtime. Get those eight hours of zzzzz. Also if you are not being too active as you travel (bus tours require a lot of sitting) take advantage of the fitness center in the hotel or find a good safe place to take a brisk walk each day.
Several travel sites suggest taking Clorox wipes to clean your seat area in the plane. I'm not that germ conscious but I do take my own sweater or soft cotton hoodie to wrap up in should I want to sleep or the plane is chilly. Actually I take one to wrap in and one to roll as a pillow. I never could get used to those blow up pillow or horseshoe pillows people use.
Another tip I found is wearing your glasses instead of contacts when you fly. You are less likely to touch your eyes with your hands. I guess I never thought of that but I usually take my contacts out after the TSA check in and wear my glasses if we're taking a flight I'm going to sleep on--less chance of my eyes getting too dry.
Did I mention wash your hands--often? That is still the number one way of fending off those nasty things that could ruin a beautiful trip with illness.