Some basic health concerns for visitors have been laid out in the Travel Section under Health and Personal Security. Please read those. However, when you live here on a long-term basis some additional precautions may be necessary.
Your primary concerns are with mosquitos, water, and food, but you must also consider access to good medical care, provisions for a healthy amount of exercise and measures to deal with a new climate as well as with the stresses of living in an unfamiliar culture.
Mosquitos are more than a nuisance. They can carry debilitating and dangerous diseases. Anti malarial prevention drugs are not recommended for long term use, and will not protect against Dengue, so you must make extra effort to avoid being bitten. Make sure the place you live is adequately protected. Screening windows is probably the best solution and in most cities companies advertise removable screens for your windows. These are plastic or fibre glass screens, held in place with Velcro tape, and are installed by the company. Price varies with quality, but the finer the mesh, the less noticeable the screens are. Steel and stainless steel mesh is also available and can be permanently attached by carpenters. This would probably work out cheaper.
Mosquito coils or electronic vaporizer can also be used, especially on patios, to keep bugs away. Mosquitos are most active in the early evening and early morning. Bite prevention must be come a regular habit. Switch to long pants and long sleeved shirts in the evening and use bug repellent if you're outdoors.
Water is another major concern since city water in most cases is not safe, and, if there are water shortages, your house or apartment may be serviced by water trucks. Do not assume it is pure. Do not drink from taps, and keep purified water in bathrooms for tooth brushing, etc. Even water for tea and coffee should be purified, unless you boil it well.
In some cities, you can get large containers of purified water delivered to your house. This is less expensive and environmentally friendlier than buying it in one litre plastic bottles. Cheaper still, you can opt for a filtering system for your drinking water. A popular filter system is AquaGuard, which uses UV light as a purifying agent. Many middle class Indian homes now use some sort of filter system, as do better hotels and some offices. If you choose to rely on filtered water, make sure the filtration system is one that removes harmful organisms. Many filters focus on improving the taste of water, rather than sterilizing it. Make sure filters are changed regularly, and the equipment is functioning properly.
For effectiveness and economy, though, you cannot beat the old fashioned concept of boiling your drinking water. The traditional recommendation is that ten minutes rolling boil (20 minutes if you are above 5,000 feet altitude) will take care of most water-borne organisms. Recently, some medical professionals have suggested merely reaching a full boil is sufficient. Establishing a regular routine ensures an adequate supply.
Food cleanliness is easier to control when you have your own kitchen, but get in the habit of washing all fruits and vegetables with soap (even many that will be peeled) as soon as they come into the house. This helps with pesticide residue as well as potential parasites. Salad vegetables are particularly tricky. Some people avoid lettuce and raw leafy vegetables completely, others wash them well and rinse with purified water.
Find a local doctor that you know and trust. Make this connection when you arrive, before you have a need. You can get recommendations from friends or colleagues. Identify a good hospital for emergency treatment as well. A number of hospital chains have been set up recently, offering better quality care. These are expensive and cater to the wealthy. You can argue about the fairness of this, but examine the facilities of the "free" government hospitals before you check in to one of them. If you do not have medical insurance from your home country, you can purchase it here from several insurance companies.
If you do have a need for a hospital stay or an operation here, make sure someone is with the patient as much as possible. Ward care may not be adequate. In addition, during an operation someone will need to be available to run to a pharmacy outside the hospital to purchase supplies as needed by the surgeons! This may sound absurd to people used to the health care systems in the West, but it is routine practice here.
Regular exercise is a necessary element of preventative health maintenance, but it can be difficult to do in crowded cities. Walking, jogging and cycling may not be practical due to the heat, pollution, or lack of places to do it. Swimming is probably the best exercise for India, but finding a good pool is difficult. Some private clubs and fancy hotels may have decent pools, as well as other exercise equipment you can use for a fee. If you enjoy jogging you may be able to find a park or an area outside the city where you can jog early in the morning before the heat and pollution levels rise.
Heat can be a problem, especially if you are being quite active. Keep up your fluid and electrolyte balances, and avoid exertion in the heat of the day. If sleeping becomes difficult because of the heat, consider air conditioning or a desert cooler. However, don't overdo the aircon. If you live in a totally air-conditioned environment, you may find yourself unable to deal with heat and humidity outside. If you live in a dry, hot climate, a desert cooler will make life much more comfortable.
Alcohol has been the bane of many an expat's life. It is almost a clichè, but alcohol related problems are easier to fall into when you're far from your normal environment, under unfamiliar stresses, and where much socializing involves drinking. Be aware of the dangers and try and avoid the pitfalls. Most major cities would have a chapter of Alcoholics Anonymous, if the need arises.
Stress is a problem for all organisms, and you may occasionally find yourself stressed in India in ways that you are not used to. Dealing with a foreign culture, with different values and ways of doing things may occasionally take its toll. So can a different climate, distance from old colleagues, family, and friends, and the loss of pleasures and ways of socializing you are used to. Even Indians returning from years abroad can feel out of place at times. Not only have their values and habits changed while they were away, but India itself has changed, dramatically in some ways.
Give yourself time. People react to being in a new culture in different ways. Most commonly, people find the first few months interesting and exciting, but 'bottom out' after a certain point. They may get depressed looking at the long months or years ahead of them and be easily angered by cultural differences. This is a normal and common reaction, so try not to overreact. It usually passes with time, as you adjust to new ways of doing things and new perspectives. Don't try and change the reality around you. Where possible, be an observer, preferably one with a sense of humour, and try not to impose your cultural judgements too quickly. Time has an amazing impact on perspective. You'll meet people who had a tough time when they first arrived in India, but who later develop a strong attachment and affection for the place.
Spouses accompanying employees assigned here may find it an extra challenge to integrate and create a satisfactory social and employment situation here. A certain amount of self-sufficiency is required, as well as initiative, and good-humour. A support system is also essential - get to know people outside of your spouses workplace. Some women have found organizations such as the Bangalore Overseas Women's Club very valuable.
Women (spouses and company employees assigned here) who have been used to jobs and social environments where they, as women, were treated relatively equally, may be dismayed at the double standard that still exists here and at the stereotypes of western women that many people still hold. Paying careful attention to how middle class and professional women dress, and being aware of different standards of appropriate behaviour will ease things all round.
Stay connected to your old life in whatever ways you can. Email is handy for keeping in touch with friends and family, and much cheaper than the telephone. (If you do like to phone home a lot, you might consider a call-back service - advertised in the back of many international magazines - to save on phone bills.)
Develop new social circles. Some cities, Bangalore in particular, have organizations that plan social events for expats and returned Indians and a local events magazine will tell you what's going on in town. You can join a film club, go to gallery openings and cultural performances, take yoga or language classes. Make it a point to interact with interesting people you meet. Don't stick only to the expat community. You will find that there is a large group of (English speaking) Indians with whom, culturally, you have much in common. Thirty to forty million Indians have grown up with English as their mother tongue and have a strong international perspective.
Plan an occasional get-away, whether for a night, at a nice restaurant, or for a weekend or longer. Short trips can be arranged to places like Singapore, Bangkok, Mauritius, or the Maldives, if you have the money. Local trips are cheaper, and could be to a hill station like Ooty or Kodaikanal, or trekking in Nepal. They could be to the beach, like Mahabalipurum, or to Kerala or Goa. Or to a game sanctuary, like Mudhumalai or Nagarhole. Plan time to visit different parts of India while you're here. Too many expat residents plan to visit the country at the end of their stay and then miss out because of unforeseen constraints. India is an amazing country, and well worth seeing.