A lot is certainly known about travellers' diarrhoea. It occurs most often in Africa, Asia and Latin America, where the risk for the individual of having an attack is between 20 and 50 per cent. Its familiar names - turista and Montezuma's revenge - show that the local adult population is usually more or less immune.
The common causes of travellers' diarrhoea are infections with bacteria and viruses. Around 40 per cent of cases are due to one bacterium, enterotoxigenic Escherichia coli; salmonella accounts for 5 per cent. In the past 20 years a whole array of diarrhoea-causing viruses has been identified - astrovirus, rotavirus, Norwalk virus - and these commonly account both for travellers' diarrhoea and for outbreaks of 'stomach trouble' on cruise ships and at holiday clubs. Tropical countries are also high-risk territories for parasites which may cause persistent diarrhoea that can last for months if the cause is not identified.
So we know the bugs that cause the problem and we know how people become infected: they swallow the microbes in food and drink and in the water where they swim (sea water contaminated by sewage is a potent cause of infection, and hotel swimming pools should not be assumed to be sterile). Bacteria and viruses survive in ice cubes and are not necessarily killed by alcohol. They thrive on food set out for buffets and they are almost inevitable contaminants of unwashed fruit and salads.
But who wants to go abroad and reject all local food and drink as potentially infected? A few cautious families with children can be seen on camp sites eating food they brought with them, and affluent businessmen eat only in five-star hotels, which are as safe in the tropics as they are in Europe. Much of the charm of a holiday, however, lies in eating in open-air restaurants and cafes in picturesque but no doubt insanitary settings.
The choices for the traveller who is not in the luxury class but can't afford to be taken ill are two: stick to the safe, boring diet - no ice, only boiled or bottled water, no salads, only food served too hot to eat - or take some antimicrobial drugs (which can only be obtained on prescription). Of these, the old standbys, the sulphonamides, give 50 per cent protection; newer antibiotics such as cotrimoxazole or ciprofloxacin give around an 85 per cent guarantee of freedom from diarrhoea. The same drugs may be used to cut short the length of an attack in someone who has not used them as a precaution. The problem - set out in detail in a recent review in Gut, the journal for gastroenterologists - is that all these drugs have side effects. They may causes rashes, thrush and - very rarely, around a one in 10,000 chance - life-threatening complications such as damage to the bone marrow and a nasty illness called the Stevens Johnson syndrome.
In the words of the review: 'Travellers who choose antimicrobial chemoprophylaxis place themselves at risk of a potentially fatal adverse drug effect while attempting to prevent what is usually a mild to moderate self-limiting illness.' Drugs other than antibiotics (such as bismuth) may be safer but they are less effective. Most of us will continue to take the risks, build up immunity to the bugs in our favourite country, and accept an occasional episode of holiday diarrhoea as an unlucky accident. The simple treatment is with lots of fluids - if necessary using sachets of salt and glucose to make up a fluid replacement solution.
For some travellers, symptoms may persist after they return home. Tropical countries are high risk areas for some intestinal parasites not found in Europe. Persistent diarrhoea may be due to amoebic dysentery, a nasty infection of the large bowel that causes internal bleeding. Amoebic dysentery does not respond to standard antibiotics, but once the diagnosis is made, treatment with specific drugs is straightforward.