Workers in Colombo, Sri Lanka, fumigate city streets as part of a campaign to curb the breeding of mosquitoes that spread dengue fever. Dengue, an acute febrile disease caused by a virus that affects tens of millions of people a year worldwide, is a particular threat in tropical areas such as Sri Lanka, and health officials there say this year's epidemic is the worst since 2000. So far this year, 146 Sri Lankans have died of dengue fever and more than 10,000 have been infected. Unlike many tropical diseases, dengue is as serious a threat in cities as it is in rural areas, and the situation in Sri Lanka is made worse by the tens of thousands of refugees crowded into displacement camps during the recent climax of the country's civil war. Officials expect the situation to worsen soon, with the beginning of monsoon season. 'We are very worried about the outbreak of diseases,' Suresh Bartlett, the director of the Sri Lanka program for the aid agency World Vision, said last week. 'When the rains come in two weeks or so, I can't imagine what conditions will be like due to the lack of any proper drainage and toilet system.' 