The average Briton spends nine days a year sleeping away from home while on summer holidays. Seventeen million of us go on package holidays and spend around £46 a day on board and lodging - a modest sum. Appreciably more is spent by those who book their own accommodation and travel.

However, whether heading to a B&B in the Mediterranean, or a five-star hotel in the Caribbean, a quarter of holiday investment is wasted because of health problems. A recent report by Boots Travel Probiotic suggests that British holidaymakers spend £391 million a year sitting on foreign loos. Nearly half, 48 per cent, of those surveyed suffered from gut problems overseas. Even one in four of those holidaying in the UK picked up something nasty.

Recent headline stories have told of the death, presumably from food poisoning, of a 71-year-old man staying at a hotel near Lake Garda in Italy (30 other tourists were also laid low); of an outbreak of Salmonella Saint Paul in America; and of unsafe foods sold by fraudsters in Britain. These stories confirm suspicions that once someone abandons home cooking they may be in for trouble.

It is now scientifically established that eating away from their usual neighbourhood is the relevant factor in causing travellers' diarrhoea. The holiday trade would prefer that everyone believed this was because emotional tensions were raised by battles at airports, anxieties over luggage, niggling doubts as to the hotel booking and the discovery that the reserved bedroom was next to a building site. Travel agents will admit that water can be responsible - not because of bacterial contamination from glasses rinsed in dirty washing-up water but, they suggest, because the geological structure of the local mountains makes a mysterious difference to the water's chemistry so that it becomes mildly laxative.

Travelling stress does undermine the immune system and lower resistance to infection, but in most cases the principal cause of traveller's diarrhoea is bugs from dirty hands of waiters, filthy, bacteria-laden dishcloths and cutting boards in unhygienic kitchens, and the organisms that live around plugholes in sinks. Most people, wherever they come from, have an immune system that is able to deal with the local bacteria. However, if they travel even 100 miles, the neighbourhood strains of E. coli and salmonella encountered are different, so that usually benign bugs upset the stranger's guts.

Most travellers understand that water may be contaminated and that salads as well as uncooked and reheated foods may carry alien bacteria or toxins, and have learnt that waiters' dirty fingernails are as, or more important than, unhygienic cooking practices, but other hazards are not fully comprehended. Although a significant proportion of travellers' diarrhoea is the result of eating unwashed fruit, only 4 per cent of tourists included this as a possible cause of trouble. Of the 750 Americans who were recently taken ill because of Salmonella Saint Paul, 100 were taken to hospital and one died. They had eaten contaminated tomatoes.

The easy but wrong way to defeat travellers' diarrhoea is to take prophylactic antibiotics. This can increase antibiotic resistance and is therefore not considered generally desirable. Holidaymakers would benefit more from building up natural resistance days before boarding an aircraft.

A good way to do this is to take probiotics, the live, “friendly” bacteria that can reinforce the billions of good bacteria already living in a healthy gut. Probiotic bacteria have the ability to change the environment in the intestines so that it becomes less welcoming to invading disease-bearing bacteria.

Probiotics are found in some yoghurts or swallowed as probiotic supplements, but however taken, two problems must be overcome. The friendly bacteria must pass through the stomach and upper gut without being damaged or destroyed by digestive juices or acids so that they settle in the gut in adequate numbers. A probiotic capsule containing 10 billion bacteria per capsule answers both demands and is now available from Boots as a travel probiotic. Seven Seas Multibionta is one of several other effective probiotic supplements, and yoghurt-type drinks such as Yakult and Actimel are also useful.

Whichever probiotic is chosen, it must be taken daily to give an adequate dose of the friendly bacteria. An additional precaution is to prepare the gut so that the best possible reception for probiotics is provided. This can be done by taking prebiotics such as Bimuno, a soluble powder for adding to hot drinks or breakfast cereals. Meticulous hand-washing, preferably with a medicated soap, is another precautionary measure.

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