Once you get above a certain population density (and India is way beyond that), you have to apply significant technology to the problem. Example: London in the 1840's, where there were repeated cholera epidemics. Their solution was to build over 100 miles of sewage line to move it closer to the ocean. That's not an option in much of India (the Ganges and other major rivers are already toxic with human waste), so they'll have to spend hundreds of millions of rupees on advanced treatment & reclamation systems.