Some people can get sick when they move to a new country. A new environment introduces one to a variety of new bacteria and pollutants. Culture shock and/or stress from moving to a new environment can also weaken one's immune system. In my experience staying healthy in a foreign country isn't any different than staying healthy in your own country.
I lived in Korea for over 3 years, but I can only remember getting sick two times, once from food in a restaurant and another from some sort of cold or flu that was caused from work related stress. I lived in Taiwan for two years and I can only remember getting sick one time from the food. I lived in China for 6 months and I only got sick one time from the food.
So if you're healthy now then you just need to maintain whatever health practices you have now wherever you go. Food will be different and your gut will be introduced to new bacteria. Staying healthy while abroad involves doing a few simple things: exercising, eating healthy foods - including lots of fruits and vegetables, and maintaining a healthy psychological state.
The water is different. I rarely drank the tap water in any of those places. If I did it was boiled. I always bought bottled water. It would be in your best interest to check with the locals and other foreigners who are already living there to find out about the water quality.
Some people say never eat street food. But I personally never got sick from it. I have eaten it all over the world. Some people say that it is not clean. That could be the case in some instances. But you can at least see the food being cooked in front of you. That's not the case in most restaurants.
Eat or drink plain yogurt. Some yogurt is loaded with sugar and additives. Find one with less sugar. Yogurt has some beneficial bacteria in it that can aid digestion. Ingesting yogurt can help your insides make a smoother transition to accommodating the new food and bacteria in a new environment.
Restaurants don't always serve fresh fruits and vegetables. So be sure to visit the local markets or supermarkets to find some fresh fruits and vegetables. Be sure to exercise. If you like to run, go to the gym or do yoga then be sure to do it in your new country. Living in Asia also provides an opportunity to learn a martial art. Taking martial arts, yoga, or any other sports classes can also be a good way to make friends. Living abroad can be a lonely experience so having a social life will help keep you healthy. Meditation is an excellent tool that will help you to reduce stress, develop your mind and maintain your overall well being.
Lastly use your judgement. If something doesn't look right or smell right then don't eat it.