My favourite place to start for travel advice is Wikitravel, here is the "stay healthy" entry for India:http://wikitravel.org/en/India#Stay_healthy
Avoiding Delhi belly
Four quick tips for keeping your stomach happy:
* Go vegetarian, at least for the first week or two. Meat spoils quickly.
* Avoid raw leafy vegetables. They are hard to clean properly.
* Avoid ice and unbottled water. Both the water in it and the way it's transported are suspect. Try to use exclusively commercially available sealed bottled water.
* Wash hands before eating, with soap. Otherwise the dirt of India's streets will find its way onto your chapatis and into your mouth. In addition, keep nails cut short and clean.
Going to India, you have to adapt to a new climate and new food. Most travellers to India will become at least slightly ill during their stay there - even Indians returning from abroad. However, with precautions the chance and severity of any illness can be minimized. Don't stress yourself too much at the beginning of your journey to allow your body to acclimatize to the country. For example, take a day of rest upon arrival, at least on your first visit. Many travellers get ill for wanting to do too much in too little time. Be careful with spicy food if it is not your daily diet.
No vaccinations are required for entry to India, except for yellow fever if you are coming from an infected area such as Africa. However, Hepatitis (both A and B, depending on your individual circumstances), meningitis and typhoid shots are recommended, as is a booster shot for tetanus.
Tap water is generally not safe for drinking. However, some establishments have water filters/purifiers installed, in which case the water is safe to drink. Packed drinking water (normally called mineral water) is a better choice. But if the seal has been tampered, it could be purified tap water. So always make sure that seal is intact before buying.
Fruits that can be peeled such as apples and bananas, as well as packaged snacks are always a safe option. Do not eat grapes unless you wash them thouroughly.
Diarrhea is common, and can have many different causes. Bring a standard first-aid kit, plus extra over-the-counter medicine for diarrhea and stomach upset. A rehydration kit can also be helpful. At the least, remember the salt/sugar/water ratio for oral rehydration: 1 tsp salt, 8 tsp sugar, for 1 litre of water. Most Indians will happily share their own advice for treatment of illnesses and other problems. A commonly recommended cure-all is to eat boiled rice and curd (yoghurt) together for 3 meals a day until you're better. Keep in mind that this is usually not sound medical advice. Indians have resistance to native bacteria and parasites that visitors do not have. If you have serious diarrhea for more than a day or two, it is best to visit a private hospital. Parasites are a common cause of diarrhea, and may not get better without treatment.
Malaria is endemic throughout India. CDC states that risk exists in all areas, including the cities of Delhi and Mumbai, and at altitudes of less than 2000 metres in Himachal Pradesh, Jammu, Kashmir, and Sikkim; however, the risk of infection is considered low in Delhi and northern India. Get expert advice on malaria preventatives, and take adequate precautions to prevent mosquito bites. Use a mosquito repellent when going outside (particularly during the evenings) and also when sleeping in trains and hotels without airconditioning. A local mosquito repellent used by Indians is Odomos and is available at most stores.
Getting vaccinations and blood transfusions in India increases your risk of contracting HIV/AIDS-even in many private hospitals.
If you need to visit a hospital in India, avoid government hospitals. The quality of treatment is poor. Private hospitals provide better service.