Many people have asked me why the outbreak of Ebola virus disease in West Africa is so large, so severe, and so difficult to contain. These questions can be answered with a single word: poverty.

The hardest-hit countries, Guinea, Liberia, and Sierra Leone, are among the poorest in the world. They have only recently emerged from years of conflict and civil war that have left their health systems largely destroyed or severely disabled and, in some areas, left a generation of children without education. In these countries, only one or two doctors are available for every 100,000 people, and these doctors are heavily concentrated in urban areas. Isolation wards and even hospital capacity for infection control are virtually nonexistent. Contacts of infected persons are being traced but not consistently isolated for monitoring.

Large numbers of people in these countries do not have steady, salaried employment. Their quest to find work contributes to fluid population movements across porous borders. The area where the borders of the three countries intersect is now the designated hot zone, where transmission is intense and people in the three countries continue to reinfect each other. Recent decisions to quarantine this area have brought extreme hardship to more than a million people — but are essential for containment.

These are only some of the many challenges to be overcome in the worst Ebola outbreak in the nearly four-decade history of this disease. The needs are enormous; the prospects for rapid containment are slim. The outbreak, in all its unprecedented dimensions, is an emergency of international concern and a medical and public health crisis, but it is also a social problem.

Now, 6 months into the response to the outbreak, fear remains the most difficult barrier to overcome. Fear causes people who have had contact with infected persons to escape from