It's always easier to become ill in a foreign country with a different climate, different food and different germs, still more so in a poor country with lower standards of sanitation than you might be used to. Most travellers, however, get through Mexico without catching anything more serious than a dose of Montezuma's Revenge. You will still want the security of health insurance, but the important thing is to keep your resistance high and to be aware of the health risks linked to poor hygiene, untreated water, mosquito bites, undressed open cuts and unprotected sex.

What you eat and drink is crucial: a poor diet lowers your resistance. Be sure to eat enough of the right things, including a good balance of protein (meat, fish, eggs or beans, for example), carbohydrates, vitamins and minerals. Eating plenty of fibre helps to avoid constipation, while peeled fresh fruit will keep up your vitamin and mineral intake although it might be worth taking daily multivitamin and mineral tablets with you. It's also important to eat enough - an unfamiliar diet may reduce the amount you eat, drink plenty of fluids and get enough sleep and rest, as it's easy to become run-down if you're on the move a lot, especially in a hot climate.

The lack of sanitation in Mexico is often exaggerated, and it's not worth being obsessive about it or you'll never enjoy anything. Even so, a degree of caution is wise - don't try anything too exotic in the first few days, before your body has had a chance to adjust to local microbes, and avoid food that has been on display for a while or not freshly cooked. You should also steer clear of salads and peel fruit before eating it. Avoid raw shellfish, and don't eat anywhere that is obviously dirty (easily spotted, since most Mexican restaurants are scrupulously clean) - street stalls in particular are suspect.

There are no required inoculations for Mexico, but it's worth visiting your doctor at least four weeks before you leave to check that you are up-to-date with tetanus , typhoid and hepatitis A jabs. Those travelling from the USA or Canada will have to pay for inoculations, available at any immunization centre or at most local clinics. Most GPs in the UK have a travel surgery where you can get advice and certain vaccines on prescription, though they may not administer some of the less common immunizations. Travel clinics can be more expensive, but you won't need to make an appointment. In Australia and New Zealand, vaccination centres are less expensive than doctors' surgeries. Most clinics will also sell travel-associated accessories, including mosquito nets and first-aid kits.

Despite all the dire warnings, a bout of diarrhoea ("Montezuma's Revenge", or simply turista as it's invariably known in Mexico) is the only medical problem you're at all likely to encounter. No one, however cautious they are, seems to avoid it altogether, largely because there are no reliable preventive measures. It's caused by the bacteria in Mexican food, which are different from (as well as more numerous than) those found in other Western diets, and is compounded by the change in diet and routine.

If you go down with a mild dose of the runs unaccompanied by other symptoms, this will probably be the cause. If your diarrhoea is accompanied by cramps and vomiting, it could be food poisoning of some sort. Either way, it will probably pass of its own accord in 24-48 hours without treatment. In the meantime, it's essential to replace the fluid and salts you're losing, so drink lots of water with oral rehydration salts - suero oral (brand names: Dioralyte, Electrosol, Rehidrat). If you can't get these, dissolve half a teaspoon of salt and three of sugar in a litre of water. Avoid greasy food, heavy spices, caffeine and most fruit and dairy products; some say bananas, papayas, guavas and prickly pears (tunas) are a help, while plain yogurt or a broth made from yeast extract (such as Marmite or Vegemite, if you happen to have some with you) can be easily absorbed by your body when you have diarrhoea. Drugs like Lomotil or Imodium plug you up - and thus undermine the body's efforts to rid itself of infection - but they can be a temporary stop-gap if you have to travel. If symptoms persist for more than three days, or if you have a fever or blood in your stools, you should seek medical advice.

Malaria , caused by a parasite that lives in the saliva of Anopheles mosquitoes, is endemic in some parts of Mexico. Areas above 1000m (such as the capital) are malaria-free, as are Cancún, Cozumel, Isla Mujeres, and all the beach resorts of the Baja and the Pacific coasts. Daytime visits to archeological sites are risk-free, too, but low-lying inland areas can be risky, especially at night. It's a good idea to take chloroquine (brand names: Nivaquin, Resochin, Avloclor, Aralen), starting one week before you arrive and continuing for a month afterwards. Chloroquine is unsuitable for sufferers of various complaints such as epilepsy and psoriasis but daily proguanil (brand name Paludrine) can be used in its place.

If you go down with malaria, you'll probably know. The fever, shivering and headaches are like severe flu and come in waves, usually beginning in the early evening. Malaria is not infectious, but can be dangerous and sometimes even fatal if not treated quickly. If no doctor is available, take 600mg of quinine sulphate three times daily for at least three days, followed by three Fansidar (available from a local pharmacy) taken together.

The most important thing, obviously, is to avoid mosquito bites. Though active from dusk till dawn, female Anopheles mosquitoes prefer to bite in the evening. Wear long sleeves, skirts or trousers, avoid dark colours, which attract mosquitoes, and put repellent on all exposed skin, especially feet and ankles, which are their favourite targets. Plenty of good brands are sold locally, though health departments recommend carrying high-DEET brands available from travel clinics at home. An alternative is to burn coils of pyrethrum incense such as Raidolitos (these are readily available and burn all night if whole, but are easy to break in transit). Sleep under a net if you can - one that hangs from a single point is best (you can usually find a way to tie a string across your room to hang it from). Special mosquito nets for hammocks are available in Mexico.

Another illness spread by mosquito bites is dengue fever , whose symptoms are similar to those of malaria, plus a headache and aching bones. Dengue-carrying mosquitoes are particularly prevalent during the rainy season and fly during the day so wear insect repellent in the daytime if mosquitoes are around. The only treatment is complete rest, with drugs to assuage the fever.

In a hot climate and at high altitudes, it's essential to increase water intake to prevent dehydration. Most travellers, and most Mexicans if they can, stay off the tap water, although a lot of the time it is in fact drinkable, and in practice impossible to avoid completely: ice made with it, unasked for, may appear in drinks, utensils are washed in it, and so on.

Most restaurants and licuaderías use purified water (agua purificada), but always check; most hotels have a supply and will often provide bottles of water in your room. Bottled water (generally purified with ozone or ultraviolet) is widely available, but stick with known brands, and always check that the seal on the bottle is intact since refilling empties with tap water for resale is not unknown (carbonated water is generally a safer bet in that respect).

There are various methods of treating water while you are travelling, whether your source is from a tap or a river or stream. Boiling it for a minimum of five minutes is the time-honoured method, but it is not always practical, will not remove unpleasant tastes, and is a lot less effective at higher altitudes - including much of central Mexico, where you have to boil it for much longer.

Chemical sterilization , using either chlorine or iodine tablets or a tincture of iodine liquid, is more convenient, but chlorine leaves a nasty aftertaste (though it can be masked with lemon or lime juice), and is not effective in preventing such diseases as amoebic dysentery and giardiasis. Pregnant women or people with thyroid problems should consult their doctor before using iodine sterilizing tablets or iodine-based purifiers. Inexpensive iodine removal filters are available and are recommended if treated water is being used continuously for more than a month or is being given to babies.

Purification , involving both filtration and sterilization, gives the most complete treatment. Portable water purifiers range in size from units weighing as little as 60g, which can be slipped into a pocket, up to 800g for carrying in a backpack. Some of the best water purifiers on the market are made in Britain by Pre-Mac. For suppliers worldwide contact:

Pre-Mac International Ltd, Unit 5, Morewood Close, Sevenoaks, Kent TN13 2HU (tel 01732/460333, fax 460222; www.pre-mac.com ).

Biting insects can be a nuisance. These include bed bugs, sometimes found in cheap hotels - look for squashed ones around the bed. Sandflies, often present on beaches, are only small, but their bites, usually on feet and ankles, itch like hell and last for days. Head or body lice can be picked up from people or bedding, and are best treated with medicated soap or shampoo.

Scorpions are mostly nocturnal and hide during the heat of the day under rocks and in crevices, so poking around in such places when in the countryside is generally ill-advised. If sleeping in a place where they might enter (such as a beach cabaña), shake your shoes out before putting them on in the morning, and try not to wander round barefoot. The sting of some scorpions is dangerous and medical treatment should always be sought - cold-pack the sting in the meantime. Snakes are unlikely to bite unless accidentally disturbed, and most are harmless in any case. To see one at all, you need to search stealthily - walk heavily and they will usually slither away. If you do get bitten or stung, remember what the snake or scorpion looked like (kill it if you can), try not to move the affected part, and seek medical help: antivenins are available in most hospitals.

Two other common causes of health problems are altitude and the sun . The solution in both cases is to take it easy. Especially if you arrive in Mexico City, you may find any activity strenuous, and the thin air is made worse by the number of pollutants it contains. Allow yourself time to acclimatize. If going to higher altitudes (climbing Popocatépetl, for example), you may develop symptoms of Acute Mountain Sickness (AMS), such as breathlessness, headaches, dizziness, nausea and appetite loss. More extreme cases may cause vomiting, disorientation, loss of balance and coughing up of pink frothy phlegm. The simple cure - a slow descent - almost always brings immediate recovery.

Tolerance to the sun, too, takes a while to build up: use a strong sun screen and, if you're walking during the day, wear a hat or keep to the shade. Be sure to avoid dehydration by drinking enough (water or fruit juice rather than beer or coffee and you should aim to drink at least three litres a day), and don't exert yourself for long periods in the hot sun. Be aware that overheating can cause heatstroke , which is potentially fatal. Signs are a very high body temperature without a feeling of fever, accompanied by headaches, disorientation and even irrational behaviour. Lowering body temperature (a tepid shower, for example) is the first step in treatment.

Less serious is prickly heat , an itchy rash that is in fact an infection of the sweat ducts caused by excessive perspiration that doesn't dry off. A cool shower, zinc oxide powder and loose cotton clothes should help

Over 40,000 cases of AIDS ( SIDA ) have been reported in Mexico, mostly in the centre of the country, and especially in the capital. It is estimated that a further 200,000 people are HIV positive. While the problem in Mexico is no worse than in many other countries, it is still a risk and you should take all the usual precautions to avoid contracting it. In particular, to contemplate casual sex without a condom would be madness - carry some with you (preferably from home; if buying them in Mexico, check the date and remember that heat affects their durability) and insist on using them. They will also protect you from other sexually transmitted diseases.

Should you need an injection or transfusion, make sure that the equipment is sterile (it might be worth bringing a sterile kit from home); any blood you receive should be screened, and from voluntary rather than commercial donor banks. If you have a shave from a barber, make sure a clean blade is used, and don't submit to processes such as ear-piercing, acupuncture or tattooing unless you can be sure that the equipment is sterile

Hepatitis A is transmitted through contaminated food and water, or through saliva, and thrives in conditions of poor hygiene. It can lay a victim low for several months with exhaustion, fever and diarrhoea, and can even cause liver damage. The Havrix vaccines have been shown to be extremely effective; though expensive (around US$150/£100 for a course of two shots), if you have a second shot within a year, protection lasts for ten years. Gamma globulin vaccines are no longer used as they are blood products and there are concerns with regard to the transmission of new variant CJD.

Symptoms by which you can recognize hepatitis include a yellowing of the whites of the eyes, general malaise, orange urine (though dehydration can also cause this) and light-coloured stools. If you think you have it, avoid alcohol, try to avoid passing it on, and get lots of rest. It's a good idea to go to a pathology lab (most towns have them) to get blood tests before seeing a doctor, who should then be consulted in order to monitor your recovery. More serious is hepatitis B , which is extremely contagious and passed on like AIDS through blood or sexual contact. Doctors advise hepatitis B vaccinations for people going on trips of over eight weeks and they are vital if you're planning to work in a medical setting or with children. Ideally three doses are given over six months but if time is short, there are other options which take one-two months with a booster given after a year.

Typhoid and cholera are spread in the same way as hepatitis A. Typhoid produces a persistent high fever with malaise, headaches and abdominal pains, followed by diarrhoea. Vaccination can be by injection or orally, but the oral alternative is less effective, more expensive and only lasts a year, as opposed to three for a shot in the arm. Cholera appears in epidemics rather than isolated cases - if it's about, you should know. It is characterized by sudden attacks of watery diarrhoea with severe cramps and debilitation. The vaccination is no longer given anywhere as it is ineffective.

Immunizations against mumps , measles , TB and rubella are a good idea for anyone who wasn't vaccinated as a child and hasn't had the diseases. You don't need a shot for yellow fever unless you're coming from a country where it's endemic (in which case you need to carry your vaccination certificate) and the polio vaccine is no longer necessary as the disease has been eradicated from the Americas.

Rabies exists in Mexico and the rabies vaccine is advised for anyone who will be more than 24 hours away from medical help. The best advice is simply to give dogs a wide berth, and not to play with animals at all, no matter how cuddly they may look. A bite, a scratch or even a lick from an infected animal could spread the disease - rabies is invariably fatal once symptoms appear so if you are bitten, assume the worst and get medical help as quickly as possible. While waiting, wash any such wound immediately but gently with soap or detergent and apply alcohol or iodine if possible. If you decide to get the vaccination, you'll need three jabs spread over a four-week period prior to travel

For minor medical problems, head for the farmacia - look for a green cross and the Farmacia sign. Pharmacists are knowledgeable and helpful, and many speak some English. They can also sell drugs over the counter (if necessary) that are only available by prescription at home. One word of warning however: in many Mexican pharmacies you can still buy drugs such as Entero-Vioform and Mexaform, which can cause optic nerve damage and have been banned elsewhere; it is not a good idea, therefore, to use local brands unless you know what they are.

For more serious complaints you can get a list of English-speaking doctors from your government's nearest consulate; big hotels and tourist offices may also be able to recommend someone. Every Mexican border town has hundreds of doctors experienced in treating gringos ( dentists , too), since they charge less then their colleagues across the border. Every reasonably sized town should also have a state- or Red Cross-run health centre ( centro de salud ), where treatment is free.

The items you might want to carry with you, especially if you're planning to go trekking, are:

Antiseptic cream

Insect repellent

Hydrocortisone/calamine lotion or any of the creams specifically designed to take the itch out of insect bites

Plasters/band aids

Water sterilization tablets or water purifier

Lint and sealed bandages

Knee supports

A course of flagyl antibiotics

Imodium (Lomotil) for emergency diarrhoea treatment

Paracetamol/aspirin (useful for combating the effects of altitude)

Multi-vitamin and -mineral tablets

Rehydration sachet