Traveling can be stressful on the body. Bad sleep, dragging around luggage, bad food, different time zones ....all this can throw off your circadian rhythm and digestive system; not to mention depress your immune system. To add to this you are going to locations where thousands of people come in and out everyday leaving germs behind them. That can be a lot for a stressed immune system to combat. The best thing you can do to prepare for a trip is to get lots of sleep and good wholesome food. Always wash your hands, use a paper towels or your sleeve to grab door handles and sanitize your seat buttons, armrests, seat belts buckles, tray tables and over head buttons. (See Passenger Shaming on Facebook for the reason). I also add to this a container of Airborne. A big boost of vitamins (especially Vit. C which boosts the immune system). When you arrive in your destination city get a good wholesome meal (proteins, grains, veg, OJ) and try to get a full nights sleep your first night.
This is not a fool proof system but it can go a long way to helping keep you healthy. Should you get sick in Europe, find a pharmacy (green plus sign) and describe your symptoms to the pharmacist, they can prescribe behind the counter meds with a quick exam (that costs little to nothing for basic stuff).