This is easier said than done. Vacations are supposed to be restful, but they rarely are. To the contrary, if you plan to take your family sightseeing in an exotic land or if you're on an important business trip where you're expected to meet with a bevy of high-powered associates at an unending series of meetings, you'll probably be working on a very tight schedule. That will likely leave you with little opportunity to lie down to rest or even to catch a decent night's sleep.
This kind of physical stress can weaken your immune system, especially if your biological clock has been thrown off by jet lag. If at all possible, schedule at least one day in your trip to adjust to the changes in clock time, so that you won't spend too much of your trip fighting the fatigue that comes from being awake at odd hours. The less stress you put on your immune system, the better the chances it can fight off the bugs that it'll encounter on your journey.