Surprised Peter, in my many trips to India (Banga and Mumbai only, sadly) I was only sick once and it was very mild, and it happened because I broke my India rules
1) other than water and biscuits eat only one meal per day
2) eat only cooked food
3) drink only bottled water and beer
Simples.
Love a dhal makhani. The glw's been promising to make us one for ages. She does make the best normal dhal, though. It's the star anise that's the secret ingredient. And the fresh coriander on top. I fry some smoked pancetta cubes and stick them in and make sure I get all the fat from the pan in too, like a piggy ghee.
You're right about the beer - Kingfisher Strong - but did you know that all bottled water in India comes the Delhi and Mumbai municipal supplies? If you're in Mumbai the water in your tap is the same as in your bottle.
On my first trip to India - 8-9 months in '94-5 - I got really bad shîts twice. Both times was when I came down from the clean air of the mountains to the heat and dirt of the plains. {Manali to Delhi, then Kathmandu to Benares/Varanasi.}