This page was originally designed for those traveling from the U.S. but much of it applies to everyone. When traveling to most foreign countries you need not be too concerned about immunizations, etc. Throughout most of Europe and the Middle East the water is safe to drink and no specific immunizations are required. But there are some countries, such as the Russian Federation and Mexico, where you probably should stick to bottled water. In Asia it may be wise to consult your person physician about getting shots for plague, etc. although incidences of people being exposed are quite rare. And, if you have any health conditions that trouble you then you should definitely consult your physician before any overseas travel.
There are vaccines that protect against hepatitis A (food- or water-borne virus) and hepatitis B (virus transmitted by bodily fluids) that your doctor may recommend. And there are certain precautions for diarrhea that can be taken. And if you are going to any place way off the beaten track you may wish to consult a physician who specializes in travel medicine. The Centers for Disease Control offers updated information on every country.
Another thing to be aware of is your dental health. If you have any loose fillings or other work that needs to be done, get it done before you leave the country. You don't want to waste valuable travel time sitting in a dentist's chair or painfully dealing with an aching tooth.
There is now a free app for iPhone, iPad & Android that stores your health records, key contacts and information on what to do in certain emergencies. The app is called iMedjetapp.
There is an organization named International Association for Medical Assistance to Travelers (IAMAT) that offers great advice for travelers and also gives a list of English-speaking doctors in many cities and countries around the world. Of course if you need medical treatment then you should seek help through local resources immediately. Trying to "tough it out" can lead to serious health problems.
The Catholic Travel Guide
Immunizations & Health Concerns for Catholic Travelers
Allergic to cigarette smoke? Sadly many more Europeans and Asians smoke than do Americans and Canadians. This used to be very unpleasant for many overseas travelers from those countries. But the good news is that more and more countries have begun to ban smoking in public places---even the pubs in Ireland! You still will not have a smoke-free environment but it should be more pleasant than in the past. Be aware that non-smoking hotel rooms are not necessarily going to be smoke-free. They just assume that you will not be smoking during your stay, but the room still may have a smokey smell to it.
If you are a smoker on a tour you will undoubtedly not be allowed to smoke on the tour bus. You should consult your tour operator to see what opportunities there will be to light up.
In China and other countries in Asia smoking is even more prevalent. It is something to be aware of when making your travel plans.