IMO, if you live / vist in the half of Delhi that has no sewerage system, open drainage, etc. you are much more at risk for any number of diseases - just like anywhere else in the world, sanitation and control of places insect vectors breed makes a huge difference. Some backpackers who want to try living on 50 rupees a day might be susceptible... but I doubt most reading this board would. Like cholera - a disease that thrives where fecal contamination occurs, and is fairly easily cured with antibiotics - conditions the very poor are apt to encounter when they have no money for medicines and open sewerage, etc.

I believe health preparation is good for any travel - paranoia spoils the experience, at best. Relating to my previous post and IAMAT, when I got to feeling badly in Agra, it was reassuring to know I had a list of physicians I could call on, though the house doc would have been OK too, in this case.