A US company Intercell is offering free holidays in Mexico and Guatemala, including airfare and accommodation, to tempt some 1,800 young people to test its drugs for one of the most common holiday afflictions, travellers' diarrhoea. In the Trek Study, willing backpackers who develop travellers' diarrhoea will be paid to attend a clinic and also Injected with a new agent to see how they react. British daily 'The Independent' reported that the holiday package includes stay in three -star hotels but can choose where they go and what they eat and drink, provided they do not stray more than three hours' travelling time from one of the centres in Mexico or Guatemala where they are required to attend for blood tests and provide stool samples if they develop an upset stomach.