E. Coli / Am I immune to it if I already got e. coli poisoning once? Why are Mexican people immune to it?

The reason I ask this is because there is an old saying about people who are travelling to mexico, etc, something like "Don't drink the water" and its known to give visitors the runs. But it does not effect people who live there , only visitors. Thats why I thought they might have an immunity from e. coli from prior exposure. I know there are other causes of waterborn illness, but I'm under the impression that when travellers get " the runs" from drinking water in various parts of the earth, it is e. coli that is causing it most of the time.
So if I already had an e. coli infection once, can I get it again?

There are literally hundreds of variants in the E.coli family, so having immunity against one varient is no protection against all the others. The locals are immune because they have been exposed to the local variety since birth, where you have not been. And there are some varients that are so aggressive in causing problems that you simply don't have time for your immune system to ward off the problem. Remember the immune system doesn't react until it is provoked, and the response can take up to 3 days to fully kick in. In 24 hours, some of the e.coli family can take down your kidneys and overwhelm the entire body. You can be dead before the immune system gets the call. And in some cases, the locals don't drink the water either, at least not if they have any choice in the matter. Unless you know for certain the local water is treated with chlorine, it's usually not a good idea to drink it. If you can smell or taste chlorine, the water is safe to drink, as there will be a zero coliform count. That is the standard for safe water. Also, it's not usually a glass of water that causes the problems. It's that same water that was used to wash your salad veggies or make the ice cubes in your drink. People tend to forget about those, although they will remember to drink bottled water. Or they brush their teeth using the water from the tap. If you are going traveling, it's always worth it to inquire about the safety of the drinking water. If the locals are drinking bottled water, then it's a good bet you should too. And use it for teeth brushing, and avoid ice in drinks. Also, shy off the fresh produce, since e.coli is killed in cooking you should only eat things that have been cooked completely. The only exception would be fruit that can be peeled, like an orange or banana.

I didn't know Mexican people were immune to it.