If I manage a trip drinking a little bit of local water without Delhi Belly, am I now fine in future?

No, you just got lucky.

Virtually everywhere in the world, tap water is fine when it leaves the processing plant, the problem is what happens between the processing plant and the tap you drink it from. If there's a leaky water pipe with another leaky sewage pipe dripping on to it, you're screwed. This is also why flooding and heavy rains often make tap water dangerous: that rusty pipe that's usually only dripping water out suddenly gets an influx of filthy floodwater/rainwater.

That said, since you've been to Bangalore several times, your intestinal flora has clearly become acclimatized to some extent and your risk of getting sick has decreased. As a random anecdote, a few years back when I was working in India, my fiancee (who was visiting for the first time) and I both got sick after sharing a dodgy meal. I got the runs but was more or less fine the next day; she got dysentery and almost ended up in the hospital.

I don't think you can make any assumptions about future trips just because of this single trip. There are just too many variables to say that you will be fine every trip in the future, e.g.

• The specific bugs in a water source can vary over time. If you were immune to some, you may not be immune to the ones next time around.

• If there has been a lot of rain, there's likely to be more contamination in the water.

• Different areas, even in the same town, may have different water sources and hence different bugs and numbers of them.

• Similarly, if there happens to be a contamination spill there will be more contamination.

• The local tap water ice cubes and drinks may have been boiled first.

You cannot build up a resistance to every water-borne disease.

Of the diseases you can build up an immunity to, there are more than one, and you have to build up immunity to each of them.

You can only build up immunity by exposure, often by getting sick and recovering, sometimes more than once.

The little bit of local water you drank is unlikely to have contained them all, and is unlikely to have been enough exposure to give you immunity to any.

The little bit of local water you drank may have even been nice clean water!

Some diseases are caused by viruses. (I'm not sure if any are common through bad tapwater.)

Some diseases are caused by bacteria.

Some diseases are caused by amoebas.

Some diseases are caused by chemicals, pollutants, or minerals, and have nothing to do with any kinds of "bugs".

Certain minerals such as arsenic and heavy metals will do more damage with prolonged exposure and will bioaccumulate in your body. This is also the case with nuclear radiation. This is quite the opposite of building up an immunity.

If any of this is scary, just drink bottled water all the time when travelling.

If you don't like to be paranoid, do what the guidebooks recommend for a particular location, or do as the locals do.

If you don't like to be ignorant, research the water quality in the specific places you're going to so you know which of the above you are dealing with.

If you're a survival type and know you're in a place where the only risk from the tap water is different intestinal flora than your own, then you can try getting used to the water.

By the way, our sister site, The Great Outdoors, has some very good questions on water safety. Oddly, they only have tags for water and water purification and not one for water safety.

Definitely just lucky. There are so many varieties, strengths and combinations of bugs at different times in different sources of water from various plants, through various pipes and taps, plus handling of food, cooking quality (does it kill the bugs?) and so on.

Consider that you can get sick from bugs occasionally at home - even after a lifetime of adapting to the local critters, it's pretty much guaranteed that at some point it 'll still get you in a foreign country.

Indeed, one study estimates that it takes up to seven years to develop an immunity of sorts, and even then it's not guaranteed.

In India, don't drink tap water unless you are in the Himalayas. It is not very polluted everywhere, but why take chances with your digestion when you can find filtered water being served in all restaurants and hotels. In some places the tap water is really not fit for drinking. We have a lot of groundwater contamination and we don't have any regular monitoring system for it either.