The thing I highly recommend anyone facing health problems is to have CONFIDENCE in yourself.
Don't get intimidated by ailments. Always drinking purified water, is just being overprotective about yourself. I'm not saying that you should drink stinky, discolored water but should have normal tap water as long as it is crystal clear, which is what you will get if you live in a good city.
Do what you like, but don't overdo it. Eating junk food is part and parcel of life, avoiding it is avoiding fun. However, there should be a limit.
Regular exercise is a MUST. Even 15~20 mins of daily exercise can work wonders.