TREKKING in Nepal, scuba-diving in Cebu or sightseeing along the Thai-Burmese border make attractive holiday propositions. But tourists often underestimate the importance of vaccinations and venture to underdeveloped parts of the region unequipped for the hazards they may encounter. ''The risk of getting something is dependent on where you are and how long you are there,'' warned Dr John Simon, a specialist in tropical diseases. ''If you are only going to be somewhere for a few days, chances are you are either not going to get it orit will not develop until you get back.'' But if you have plans to travel around a country for more than a week, you would be well-advised to spend some time with your doctor learning about the precautions you need to take. Most doctors recommend patients join a medical evacuation group if they are visiting obscure places - falling ill or having an accident in a remote part of Cambodia can be fatal unless quality medical treatment is close at hand. Companies such as Asia Emergency Assistance or International SOS Assistance provide emergency evacuation services to members. But pre-holiday immunisation can go a long way towards preventing life-threatening diseases. Dr Simon listed some infectious diseases and what could be done to minimise the risk of catching them. Cholera: a severe disease that can result in death through dehydration, it is spread through drinking contaminated water. The cholera vaccine did not work, Dr Simon said, but may still be required by some border authorities. The best way to prevent it is to be especially careful about the water you drink. Ensure it is either boiled or use water purification tablets, brush your teeth with beer if necessary and open the seals on bottled water yourself. Also, avoid ice. Diphtheria: still common in Third World countries, this disease affects the throat, heart and nervous system. Vaccines against the disease should be taken every 10 years as long as you visit developing countries. Hepatitis A: spread through shellfish, this is rarely a fatal illness but leaves you feeling nauseous and fatigued. A new vaccine, Havrix, developed by Smith Kline Beecham lasts at least 10 years. It is now licensed for use in Europe, Singapore and Hongkong. Hepatitis B: very common in Southeast Asia, where about 10 per cent of the population are carriers, the disease is spread by sexual contact and contaminated blood. About 30 per cent of carriers will develep cirrhosis of the liver or liver cancer. Doctors believe immunisation against hepatitis B to be the single most important one for people living and travelling in the region. But they warn immunity only develops six months after inoculation and a booster should be given every five years. Japanese B encephalitis: quite uncommon but potentially lethal, it has a 30 per cent death rate. It is spread by rice-field breeding mosquitoes, so those travelling to Thailand, Burma, India, Indonesia, the Philippines and southern China should be careful. The vaccine is safe, although a booster dose is needed every 18 months. Polio: this causes paralysis of respiratory and other muscles and can often lead to death. Very common in developing countries so doctors recommend an oral vaccine, which is painless. Rabies: official statistics indicate one per cent of the dogs on the streets of Bangkok are rabid, and the disease is common in all of Southeast Asia and the Indian sub-continent. Vaccines are compulsory immediately after being bitten by any animal, although in some developing countries, even these are dangerous and have serious side effects. Ensure the vaccine you are getting is safe, or fly to a place where you can be given a safe vaccine. Even if you have had a rabies shot before being bitten, you still need one afterwards. Dr Simon said he had often seen patients bitten by monkeys they were petting in Bali and warned there was no way of knowing if an animal was rabid or not. Tetanus: this is caught from a deep wound and results in lock jaw and muscle spasms and can cause death. Travellers around the region should get a booster every 10 years. Typhoid: this is spread from contaminated food and can cause a high fever and bleeding of the intestine. A new oral vaccine can help prevent this but booster courses are necessary every few years. Yellow fever: spread by mosquitoes mostly in certain parts of Africa and south and central America. Consult your doctor for a vaccination, which lasts for 10 years. Malaria: drugs such as Lariam are being recommended by doctors particularly for trips to Vietnam, Laos, Burma, Papua New Guinea and parts of Thailand. Combinations of drugs such as Chloroquine and Proguanil are being recommended for India, China, Malaysia, the Philippines and Indonesia. Because of some of the side effects associated with anti-malaria drugs, they are not necessary for trips to sophisticated resort areas such as Bali, Phuket, Pattaya or Cebu. Dr Simon advised patients to generally stay away from raw foods such as salads and said fruit should be peeled and washed before eating. He said everything else should be well cooked. Avoid under-cooked pork because of the risk of parasites or tape worms. ''People can travel around the region for a month and not become ill if they take really good measures,'' he said. ''They should take especially good care in mosquito-infested areas because of diseases like dengue fever, for which there is no vaccine. ''Doctors can help patients to put a medical travel bag together, but they should be fully aware of the side effects. Doctors should go through what to use under certain conditions in great detail but more importantly they should provide advice on when not to use something.''