Liberia

A few months ago the U.S. Centers for Disease Control and Prevention predicted that up to 1.4 million people in Liberia and Sierra Leone could become infected with Ebola by mid-January. In a recent address to the Senate, CDC director Tom Frieden said that worst-case scenario would not pan out.

That is partly because health care workers in the Ebola hot zone are engaged in a battle to contain the epidemic. It is also because of assumptions about human and viral behavior that are built into the mathematical models used to predict the spread of infectious diseases. Assumptions are inherent in these models. “You take islands of data from different places and build bridges of assumptions that link up all these islands,” says Martin Meltzer, senior health economist at the CDC. Meltzer’s model, which predicted the 1.4 million infections in Liberia and Sierra Leone, worked on the assumption that things would not improve. “Our forecasts are based on the idea that nothing will change,” he says.