Travellers' diarrhoea (TD) is rather defined by circumstances of acquisition than by specific microbial agents. TD is usually defined as the passage of 3 or more unformed stools in a 24-hour period, or any number of loose stools if accompanied by abdominal pain, fever, nausea or vomiting. TD is the most frequent syndrome among travellers in most of the visited regions and affects 20–60% of travellers . Some authors have described that 8% of travellers seek medical care upon their return; of these, one third reports diarrhoeal diseases [2–5] TD typically occurs during the first week after arrival, is often self-limiting, and lasts three to four days. Only approximately 2–3% of TD persists longer than a month [1, 6]
Efforts to determine the etiology of travellers' diarrhoea in returning travellers encounter several difficulties. Most cases of travellers' diarrhoea are relatively mild and self-limiting, and the patient may not visit a doctor to report it. However, if the patient is ill enough to see a doctor, stool samples are often not obtained for laboratory confirmation. And finally, if a sample is taken and analyzed, it may be impossible to identify a responsible organism. In fact, it has been estimated that only 1 in 136 cases of gastrointestinal infections in the UK is reported to routine surveillance systems . The most common cause of TD worldwide is enterotoxigenic Escherichia Coli (ETEC), which induce watery diarrhoea associated with cramps and with low grade or absent fever . ETEC infections are common when there is a breakdown in sanitation, which is often the case in developing countries . Other bacterial etiologies are Campylobacter (jejuni, coli), Salmonella, Shigella, Vibrio cholerae, V. parahaemolyicus, V. vulnificus, Yersinia enterocolitica, and Clostridium difficile [1, 8]. Because most cases of cholera are mild or moderate [1, 9–11]., one part of TD contracted in cholera-endemic or epidemic countries may be cholera .
The most important determinant of risk is the travel destination. Regional differences in both the risk and etiology of diarrhoea divide the world into three grades of risk (high, intermediate, and low). High-risk areas include most of Asia, the Middle East, Africa, and Central and South America. Approximately 50,000 daily cases of TD are estimated among the 50 million people travelling to developing countries. More temperate regions involve seasonal variations in diarrhoea risk. In South Asia, for example, much higher TD attack rates are commonly reported during the hot pre-monsoon months 
TD occurs equally in males and females, and is more common in young adults than in older people. Others risk factors for TD include anti-acid medications, achlorhydria, hypoclorhydria, gastrectomy, type O blood, or immune deficiency  In short-term travellers, bouts of TD do not appear to protect against future attacks, and more than one episode of TD may occur during a single trip. For travellers to high-risk areas, several approaches may be recommended which can minimize, but never completely eliminate, the risk of TD. The usual recommendations about basic hygiene are usually quickly forgotten, and followed only by a small number of travellers. Therefore, besides these useful recommendations, complementary actions for controlling these diseases -mainly transmitted by contaminated water and food- should be considered 
In Spain, preventive care of international travellers (health education and vaccination) is mostly performed through a network of 52 public International Vaccination Centres (IVC). This network was visited by a total of 188,445 and 204,985 travellers during 2005 and 2006, respectively . IVC belong to a state-based network, which performs travellers' care only. The consultations peak is from the month of May to September. By late June 2005, the oral vaccine Dukoral® (whole-cell/recombinant B-subunit cholera vaccine, WC/rBS) was marketed in Spain subsequent to its authorization in the European Union for the prevention of cholera, in April 2004 . Dukoral® had previously been authorized in another 25 countries for the indication of both cholera and ETEC-related diarrhoea, except in Australia, where it was authorized for cholera prevention only . Several studies have evidenced the efficacy of WC/rBS vaccine for protection against diarrhoea caused by LT-related ETEC [17–19].
The objectives of this article were:
To assess the effectiveness of WC/rBS cholera vaccine in the prevention of diarrhoea in subjects travelling to cholera endemic-epidemic zones and having attended the Spanish network of IVC.
To perform economical (cost-effectiveness and cost-benefit) analysis of WC/rBS vaccination in subjects travelling to high-risk zones of TD, as compared to non-vaccinated subjects with a similar destination.