Dengue or hemorrhagic fever affects millions of people worldwide every year.  In the past few decades, the incidence of the disease has increased as the range of dengue-carrying mosquitoes has expanded. In places with poor access to health care treatment, as many as 7% of people who get the disease can die.  Scientists know that epidemics of dengue fever wax and wane.  What scientists haven't been able to do is predict the severity and timing of epidemics.

New research at the University of Georgia could help public health officials design models to forecast outbreaks. Study author Helen Wearing says that because dengue is actually a disease with different strains, or serotypes, it's more difficult to achieve immunity to the disease. "So, if you get dengue -- say (for example), serotype one -- you're not then immune to all the other serotypes," she explains. "So you can actually get multiple dengue infections throughout your life, and secondary infections. So when you get dengue, say, a second time, there's an increased risk of getting a more severe form of the disease."

Wearing looked at data from the Department of Public Health in Thailand about epidemics of dengue over the past 4 decades.  What the data showed is that epidemics of the individual serotypes appear to recur every 8 to 10 years. However, Wearing notes that when one looks at dengue incidence in general, epidemics actually happen more frequently.  She says it takes about three years for people to lose whatever short-term immunity they acquire from getting the disease.