Peoples' bodies get used to the normal organisms and bacteria found in their local water. These are things your body can handle. Your body is NOT used to strange organisms found in water from other parts of the world. It's not that that water is "bad" - it's just that it's different. Your body is very sensitive to strange organisms, and tries to fight off anything strange. Your body therefore gets tummy issues and flu symptoms during that battle.
The best way to prevent this battle from beginning is to make sure that the water you drink is clear from any strange bacteria. There are two ways you can do this. One is to drink only bottled water, which is filtered before it is bottled. The second is a filter, simply filter the water before you drink it.
Bottled water is a nice option - but it is expensive. Whether you buy it at the supermarket and lug it home, you're going to pay high prices for that water. In some cases bottled water costs more, per litre, than gas does! You could drink 1,000 litres of tap water for what you pay for a bottle of bottled water. Not forgetting the environmental impacts on the processing of plastic bottles.
Is it better?