Every couple of days, the World Health Organization (WHO) issues a “situation update” on the Ebola epidemic, with new numbers of cases and deaths for each of the affected countries. These numbers―9216 and 4555 respectively, according to Friday’s update―are instantly reported and tweeted around the world. They’re also quickly translated into ever-more frightening graphics by people who follow the epidemic closely, such as virologist Ian Mackay of the University of Queensland in Brisbane, Australia, and Maia Majumder, a Ph.D. student at the Massachusetts Institute of Technology in Cambridge who visualizes the data on her website and publishes projections on HealthMap, an online information system for outbreaks.
But it’s widely known that the real situation is much worse than the numbers show because many cases don't make it into the official statistics. Underreporting occurs in every disease outbreak anywhere, but keeping track of Ebola in Guinea, Liberia, and Sierra Leone has been particularly difficult. And the epidemic unfolds, underreporting appears to be getting worse. (“It’s a mess,” Mackay says.)
So what do the WHO numbers really mean—and how can researchers estimate the actual number of victims? Here are answers to some key questions.