Nothing ruins a trip quicker than falling ill. I take a number of measures to try and keep healthy whilst travelling (mainly because I almost always pick something up on long flights…).
Some suggestions:
Have something to boost your immune system, start taking it before you depart and keep it up for at least the start of your trip. I use Turkey Tail mushroom supplements to boost my immune system, but you can take any of the many other supplements like echinacea.
If you can afford it, get something like AG1 travel packs. AG1 is a daily greens supplement. It isn’t cheap, but it is great for making sure that you have all of your nutritional bases covered when you may not be following the healthiest diet on the road. For a recent trip, I found some “Supergreens and Superfoods” supplement tablets on Amazon, which were much more reasonable.
Always carry lots of water
I generally stock up on trail mix so that I have something nutritious to snack on, even if I am not near any restaurants.
Whenever you are in a fast food restaurant, stock up on napkins - they always come in handy
If you are travelling to high altitude (which covers a great deal of the western side of the USA), always have sun cream. The UV at high altitude can be quite extreme even if it doesn’t seem hot.
Obviously ensure that you have travel insurance for emergencies
Get yourself a UK Global Health Insurance Card - this replaces the EHIC card since the UK left the EU - meaning that you can access healthcare in the EU and some other countries. It is free and you can apply online.