It's not always easy to get safe drinking water when you're on the road. Sure, there's always bottled water but the cost of that can mount up and unscrupulous vendors have even been known to refill old bottles from the tap and sell them to unwary travellers.
You can try purification tablets but in my experience the stuff they produce is almost undrinkable. I often boil water and refill my own bottle. But easier still is a Steripen which knocks out the bugs with ultraviolet light and doesn't affect taste.
I tried a Steripen a couple of years ago and it worked fine. And now there's a new model which is easier to use, even smaller, lighter and requires two batteries instead of the previous four. It's a bit expensive - ranging from $250 to $350 depending on the model - but it's said to be capable of producing 8000L of water so if you travel in third world countries a lot it's probably worth your while.
You can buy the Steripen from most outdoor stores and travel medical specialists.