It's all psychosomatic. The hard of thinking go to India convinced they are going to get a stomach bug, and, hey presto! 8 hours later they're ****ting rusty bum gravy at high velocity.
Eat some yoghurt, go outside your hotel and eat street food with the locals and enjoy the experience.
When I went to Egypt as a kid we were warned not to eat or drink anything we bought locally. I said "screw that, I'm invincible" and ate some ice-cream with two other kids. All three of us became ill with the runs. In Guatemala after I bravely drank mosquito-infested water in the jungle I had dodgy insides for months.