An outbreak of bird flu in India in 2008 prompted authorities to temporarily ban the sale of poultry. Diptendu Dutta/AFP/Getty Images hide caption

itoggle caption Diptendu Dutta/AFP/Getty Images

Employees of a petroleum company in Liberia help to curb Ebola's spread via a public health awareness campaign Monday. West Africa is facing its first Ebola outbreak, so questions abound. Abbas Dulleh/AP hide caption

itoggle caption Abbas Dulleh/AP

The usual suspect: Bats harbor dozens of deadly viruses, such as rabies and influenza. Several studies suggest that bats may also carry Ebola. Tyler Hicks/Getty Images hide caption

itoggle caption Tyler Hicks/Getty Images

Television trucks converged on Mount Sinai Hospital in New York on Monday after it announced that it was screening a patient for Ebola virus. Stan Honda/AFP/Getty Images hide caption

itoggle caption Stan Honda/AFP/Getty Images

When Sule Koroma's sister Saudatu died of Ebola, his house in Freetown was put under a 21-day quarantine. The government sent two policeman to guard the house. Tommy Trenchard for NPR hide caption

itoggle caption Tommy Trenchard for NPR

Dr. Kent Brantly, of Fort Worth, Texas, was treating Ebola patients in Monrovia, Liberia, when he himself became infected with the virus. Courtesy of Samaritan's Purse hide caption

itoggle caption Courtesy of Samaritan's Purse

Red Cross volunteers prepare to bury the body of an Ebola victim in Pendembu, Sierra Leone, early this month. Tommy Trenchard for NPR hide caption

itoggle caption Tommy Trenchard for NPR

Usman (right), 7 months, and Abdullah (left), 18 months, are held by their mothers while they wait to receive the polio vaccine at the Jalozai refugee camp near Peshawar, Pakistan. Daniel Berehulak/Getty Images hide caption

itoggle caption Daniel Berehulak/Getty Images

During nationwide polio campaigns, hundreds of thousands of health workers go door to door, giving children two drops of the polio vaccine. Anadolu Agency/Getty Images hide caption

itoggle caption Anadolu Agency/Getty Images

Medical workers treat Ebola patients at the Eternal Love Winning Africa hospital in Monrovia, Liberia. Three workers at the hospital, including Dr. Kent Brantly (left), have tested positive for Ebola. Courtesy of Samaritan's Purse hide caption

itoggle caption Courtesy of Samaritan's Purse

Dr. Kent Brantly (right) of Samaritan's Purse gives orders to treat Ebola patients through the doorway of the isolation ward in Monrovia, Liberia. Courtesy of Samaritan's Purse hide caption

itoggle caption Courtesy of Samaritan's Purse