Few people have serious health problems when they visit India. If you are careful about the water you drink and what you eat, and stay in reasonably clean hotels, you have a good chance of remaining healthy.
I am suggesting that the best is to only drink water from bottles which are with a complete plastic seal from a shop. It is a good idea to use bottled or purified water even when brushing your teeth.
If you take water from a tap, which is claimed to be drinking water you should still boil the water. For this reason I travel with a metal cup and a small electric spiral water heater. Often, fruit juices are mixed with tap water. Avoid the local street vendors and be careful of ice, as you have no idea of the source of the water. Green coconuts are a good source of clean water. Green coconuts are available in West Bengal, Orissa, Mumbai, and South India.
With the same spiral heater you can heat up a bucket of water in about 30 minutes and take bath.
Medical professional advice to take inoculations against Hepatitis A, tetanus, typhoid and malaria and to take pills with you against malaria.
Personally I never received any inoculation, but one time I came back with jaundice en once I almost left my body because of malaria. After this experience I am always taking anti-malaria pills with me, which are easily available in Indian medical stores. There are chemical pills with nasty side effects and there are Ayur-veda medicines which according to some are also usable, but not sure to be reliable.
If you plan to take pills to prevent malaria, begin taking those two weeks before departure and continue to take them for at least six weeks after you return. The sure way to avoid malaria is to avoid being bitten by malaria-carrying mosquitoes. If you are careful, use insect repellent (brought from the West-the ones in India are not as effective), and sleep under a mosquito net, this way you can avoid being bitten by mosquitoes.