The redder the zone you live in on the map at right, the greater your risk for contracting dengue, a painful viral disease that new research finds is far more prevalent than previously thought.

There is no vaccine, nor are there drugs to treat dengue, which is spread by mosquitoes and known colloquially as "breakbone fever" for the aches it produces. Patients with severe dengue recover on their own with medical support, but complications occasionally cause death. There are four types of the virus. Infection makes you immune to that particular dengue type for life, but infection with a second type increases the likelihood of severe disease.

"Dengue is one of the few infectious diseases increasing its global spread and the number of cases annually," says Jeremy Farrar, a clinician and infectious disease specialist at the University of Oxford Clinical Research Unit in Ho Chi Minh City, Vietnam. For public health interventions to be effective, "it's crucial we understand where the disease is today and have an understanding of where it may be tomorrow." But knowledge of dengue was based on incomplete reporting, a lack of data on mild cases, and crude approximations that produced "back-of-the-envelope estimates," Farrar says.

To get a better handle on the spread and incidence of the disease, Farrar and epidemiologist Simon Hay of the University of Oxford in the United Kingdom formed a team that compiled 8300 reports of dengue infections and considered new evidence on risk factors, such as population growth in urban areas where the virus-carryingAedes aegypti mosquito thrives and can bite several humans in rapid succession. Then, using new modeling techniques, they concluded that in 2010, dengue sent 96 million people to clinics or caused them to miss school or work, while another 294 million had mild or asymptomatic infections. The 390 million total is more than three times the 50 million to 100 million annual dengue cases now assumed by the World Health Organization (WHO), the researchers report online today in Nature.

Duane Gubler, a dengue expert at the Duke-National University of Singapore Graduate Medical School in Singapore who was not involved in the study, is relieved. "Finally, there is good evidence-based data that confirms what I have been saying for years, that the 'low ball' WHO figures grossly underestimate the true burden of dengue." If anything, Gubler suspects the estimate is low because, among other things, the team drew on some community studies focused on childhood infections when rates among adults can be higher. "However, I think it is an excellent article that will contribute greatly to our understanding of this disease," he adds.

The new findings won't impact the clinical management of dengue. But the authors believe that their more precise mapping of the disease's distribution and the better estimate of total infections will help public health officials plan mosquito control efforts and future vaccination campaigns and to measure their impact. And, Farrar says, the team intends to continue to refine what he says is still an estimate of the disease burden, using better data as it becomes available.