Unless you're absolutely sure, always assume that the water is untreated. Use water from containers with a proper seal, or purify it by boiling or treating it chemically. The water in Calcutta is extremely polluted. Take special care, because gastric disease is endemic here. If mosquitoes do bite you, try not to scratch the bite as infection and tropical ulcers can easily set in.
Aids is a problem in India, as in most places, so take the necessary precautions.
You may want to carry your own syringes and transfusion kit. Crime levels are well below those of Western countries and violent crime is unusual. However, tourists are always obvious targets for thieves. Bihar is notorious for its absence of law and order. Kidnappings, banditry, murder and rape are common occurrences.