In an audio commentary entitled "A Missionary's Expectation," when Elisabeth Elliot is asked to advise potential missionaries, one of her main pieces of advice is, "don't worry about your health." This is, of course, biblically-based (Mt. 6:25). The whole broadcast is actually quite insightful.
However, I believe that thinking about something is not the same as worrying about it. Of course you are going to plan and problem-solve, even while you know that God is in control.
Elisabeth Elliot's advice is to wash your hands, and that's what I would do, too. Wash your hands, boil your water, and trust in the Lord. You are going to face physical discomfort living in a new country. But since many health problems are caused by stress, stressing about the stress is only going to make matters worse. Once I realized that a lot of problems like stomachaches were caused by stress, it made me want to take better care of my spiritual and emotional health. I have a cluster of white hairs from my first year in Russia, but haven't gotten any new ones since then. :)