Another Indian Mystery: Solved?

Another great Indian mystery:

Why does Indian food (sometimes) give people diarrhea?

I had always assumed this was a Western-stomach-not-used-to-Indian-food thing. But yesterday Mudit and Shatabdi, Mudit's wife and another coworker and friend, were complaining about how the office food gave them diarrhea. I was shocked. Indians were having diarrhea too!

Shatabdi explained why the office food was giving us diarrhea: the spices were not fully cooked. Apparently, in order to stave off diarrhea, spices are supposed to be cooked (according to Shatabdi, usually fried) before being added to the food. But the cook at work was just throwing in raw spices after the rest of the food had been cooking for a while, and with not enough cooking time left to fully cook the spices.

So the next time Indian food gives you diarrhea, it's likely that the spices were thrown in raw. But I'm a bit skeptical that this is the reason every time. After all, if one throws in the spices at the same time as the rest of the food, shouldn't the spices be fully cooked? I'm sure there are other reasons behind Indian food-induced diarrhea. For me, this mystery is not yet completely solved.