“More than six months have passed since Ebola was first identified in West Africa, and the scale of the crisis continues to grow. Over the last few weeks cases and deaths have increased significantly in the two most affected countries – Liberia and Sierra Leone – setting the stage for even more explosive growth in the weeks and months to come if further action is not taken immediately. The sense of urgency in the face of an outbreak that has grown “out of control” has been palpable among public health leaders and politicians alike, including at the United Nations last week.  Some of this urgency has been driven by stark new data and modeling projections just released by the WHO and the U.S. Centers for Disease Control and Prevention (CDC).  To help shed light on these new data, we take a deeper look at several key measures of the epidemic’s impact including estimates of current cases, prevalence and death rates from Ebola, as well as a consideration of the future projections of Ebola’s burden in the months to come. How Many Cases? The global health community relies on the World Health Organization (WHO) as the authoritative source on current Ebola case numbers.  WHO has published updates and situation reports about the West African Ebola epidemic, collected from the country governments, agencies, and organizations working on the ground, approximately every week since August. Even though they represent the “gold standard” of the moment, the WHO numbers cannot be taken at face value.  As WHO itself has stated, its numbers are “vast underestimates,” and the organization believes the true number of cases is two to four times greater than the official reported numbers because many cases go undetected or uncounted for a variety of reasons.  What Share of the Population Has Already Been Affected? Recognizing these limitations, we used these data to calculate the cumulative prevalence of Ebola (the percent of the population that has been infected to date) in Liberia and Sierra Leone as of September 22.  Based on officially reported numbers from WHO, cumulative prevalence across both countries stands at 0.049%.  Liberia has been hardest hit with an estimated cumulative prevalence of 0.07%, more than twice the rate for Sierra Leone.  For illustrative purposes, if we take into account the underreporting factor as estimated by WHO by tripling the officially reported case numbers, the cumulative prevalence across both countries would be 0.147%; in Liberia alone, the adjusted prevalence would be 0.22%, or approximately one in every 500 people having become infected already (see Table).  Moreover, the estimated number of new cases is rising exponentially.”