Studies by specialists in Travel Medicine have shown that:
-- Bottled water isn't always safe,
-- Water filters have some little-known loopholes or pitfalls,
--People who are very careful and knowledgeable often get just as sick as the rest of the travelers. However, there are ways of changing this.
Illnesses contracted while travelling can disrupt or ruin a tour. It happens all too often: yet it is for the most part very preventable.
One thing getting in the way is that people assume they know everything they need to know. So they lack the humility/openness to learning. They don't seek out new or better information.
Or they rely on less-than-accurate, less-than-complete information and sources.
There is a lot to learn. I attended medical school before deciding on another course, and have read from a variety of good sources -- and I'm still learning important new points in this area. It is so easy to think one has got it down. It's easy to get complacent, but it helps to snap out of it.
There is excellent information to be found from this site (authored by an exceptionally well qualified MD who has a professional background both in the field, or medical specialization, of Travel Medicine, and in the field of Infectious Diseases, and is used by Lonely Planet for his expertise in these areas), and there is valuable additional information in his carefully selected links (to CDC and WHO travel information, for example),