Airline catering apart, one of the great pleasures of travel is the oppor- tunity to sample new foods. Unfortunately, the aphorism 'Travel broadens the mind and loosens the bowels' holds true for the majority of travellers.

A huge variety of micro-organisms cause diarrhoeal illness, with or without vomiting, and these are usually ingested with food or water. Food may carry other health hazards - unpasteurised milk and milk products transmit brucellosis in the Middle East and parts of Africa, and raw fish and crabs harbour a number of unpleasant worm and fluke infections. Even polar explorers face hazards - the liver of carnivores such as polar bears and huskies causes human illness due to Vitamin A poisoning.

Some basic rules

Although it is impossible to avoid infection entirely, the risk can be reduced by following some simple rules. The apparent prestige and expense of a hotel are no guide to the degree of hygiene employed in its kitchens, and the following guidelines apply equally to luxury travellers and those travelling rough.

Assurances from the local population (including long-term expatriates) that food is safe should not be taken too literally. They are likely to have developed immunity to organisms commonly present in their water supply. Sometimes it is impossible to refuse locally prepared food without causing severe offence, and invitations to village feasts will need to be dealt with diplomatically.

The major sources of external contamination of food are unclean water, dirty hands, and flies. Pay scrupulous attention to personal hygiene, and only eat food with your fingers (including breads or fruit) if you have thoroughly washed your hands. Avoid food handled by others who you suspect have not been so careful with their hands - and remember that in many countries toilet paper is not used.

Water

The mains water supply in many countries is contaminated with sewage, while streams, rivers, lakes and reservoirs are freely used as toilets and for personal bathing and washing clothes. The same water may be used for washing food (especially salads and fruit) and may also be frozen to make ice cubes for drinks. Water should always be boiled or treated before drinking or use in the preparation of uncooked food (detailed advice is given in the chapter on Water Purification).

Hot tea or coffee are usually safe, as are beer and wine. Bottled water and carbonated drinks or fruit juice are not always safe, although the risk of adulteration or contamination is reduced if you keep to internationally-recognised brands. Insist on seeing the bottle (or can) before it is opened, thus confirming that the seal is tight and the drink has not been tampered with.

If you have any doubts about the cleanliness of plates and cutlery, they can be rinsed in a sterile solution such as tea or coffee, or wiped with an injection swab. If this is not feasible, leave the bottom layer of food on the plate, especially if it is served on a bed of rice. If drinking utensils appear to be contaminated, it may be preferable to drink straight from the bottle.

Food

Food that has been freshly cooked is the safest, but must be served really hot. Beware of food that has been pre-cooked and kept warm for several hours, or desserts (especially those containing cream) that have been inadequately refrigerated after cooking. This includes many hotel buffets. Unpasteurised milk or cheese should be avoided, as should ice cream. Food that has been visited by flies is certain to have been contaminated by excrement and should not be eaten.

Salads and peeled fruit prepared by others may have been washed with contaminated water. In some parts of the tropics, salads may be highly contaminated by human excrement used as fertiliser. Salads and fruits are best avoided, unless you can soak them in water that you know is clean. Unpeeled fruit is safe, provided that you peel it yourself without contaminating the contents. 'Wash it, peel it, boil it or forget it' remains excellent advice.

Shellfish and prawns are particularly high-risk foods because they act as filters, concentrating illness (they often thrive near sewage outfalls). They should only be eaten if thoroughly cooked, and I recommend resisting the temptation altogether. Shellfish and prawns also concentrate biological toxins at certain times of the year, causing a different form of food poisoning. Raw fish, crustaceans and meat should always be avoided.

Hot spices and chillies do not sterilise foods, and chutneys and sauces that are left open on the table may have been visited by flies. Be cautious with chillies: they contain capsaicin which is highly irritable to the bowel lining. Beware of trying to impress your hosts by matching their consumption of hot foods.

Alcohol

The temptation to over-indulge starts on the aeroplane, but in-flight alcohol should be taken sparingly as it increases the dehydration associated with air travel and worsens jet lag. Intoxicated airline passengers are a menace to everybody, and drinking impairs your ability to drive on arrival.

In hot countries, beware of dehydrating yourself with large volumes of alcoholic drink. Alcohol promotes the production of urine and can actually make you more dehydrated.

Excessive alcohol consumption promotes diarrhoea, and prolonged abuse reduces the body's defences against infection. The deleterious social, domestic and professional hazards of prolonged alcohol abuse are well recognised problems for expatriates.