Tens of thousands of tsunami survivors are at risk from diseases spread by dirty water, mosquitoes and crowding, and the best medicine is large quantities of clean water, officials of the World Health Organization said yesterday.

　　While no epidemics have been confirmed in the vast coastal areas devastated by the tsunamis on Sunday, the officials said they were most worried about diarrheal diseases - cholera, typhoid fever and shigellosis - as well as liver diseases like hepatitis A and E. Those diseases are caused by bacteria or viruses in contaminated drinking water or food, in sewage and among people who lack clean water to wash their hands.

　　Health organizations like the W.H.O. and Unicef recommend that each person be given about five gallons of clean water a day. Dr. David Nabarro, the director of crisis operations for the W.H.O., said in a telephone interview from its headquarters in Geneva that water shortages had already occurred in the Maldives and Sri Lanka, and that tanker trucks would be needed to provide clean water.

　　In addition, water-purifying tablets are being rushed into the affected countries, along with medicines to treat the dehydration that can result from diarrhea.

　　Another hazard to drinking water is contamination of wells by salt water from the tsunamis. Martin Dawes, a regional spokesman for Unicef in Colombo, Sri Lanka, estimated that 1,000 drinking-water wells in the country's hard-hit eastern region had been contaminated and would have to be pumped out.

　　"At the moment," he said, "the water people don't have the right kind of pumps to rescue the wells." He added that his agency was seeking pumps or money to buy them.

　　Mr. Dawes said Unicef had also bought about 20 million gallons of drinking water in 1,500-gallon barrels, enough for 100,000 people, and was expecting them to be delivered to the affected areas on Thursday.