I don't know, but I suspect you can build an immunity to it with small exposures. And I suspect that taking antibiotics may increase your risks of illness to it.

I've never gotten sick drinking from clear rocky bottom streams here or in the Sierras. I was taught to drink from running water, not standing pools, and to consider the source.

I did get a tad sick in Cozumel after about a week. It only lasted about three days, but it got me. Everybody there drank bottled water, even the locals. It was probably eating fresh fruit and veggies that got me. I learned that from the locals too. I've been to Mexico a couple more times but I haven't had a bout with it since.