Dr. Rick Sacra, 51, has been working on and off in Liberia for 15 years. He went back to Monrovia in August to help deliver babies. It's still unknown how he caught Ebola. Courtesy of SIM hide caption

itoggle caption Courtesy of SIM

PCR tests like this can tell if a virus is an enterovirus, but they can't ID the new virus that has caused a surge in serious respiratory infections. BSIP / Science Source hide caption

itoggle caption BSIP / Science Source

13-year-old Will Cornejo of Lone Tree, Colo., recovers at Rocky Mountain Hospital for Children in Denver from what doctors suspect is enterovirus 68. His parents found him unconscious on the couch and called 911. He was flown to Denver for treatment. Cyrus McCrimmon/Denver Post/Getty Images hide caption

itoggle caption Cyrus McCrimmon/Denver Post/Getty Images

A security man takes visitors' temperatures Wednesday at the Transcorp Hilton hotel in Abuja, Nigeria, about 400 miles north of Port Harcourt. Afolabi Sotunde/Reuters/Landov hide caption

itoggle caption Afolabi Sotunde/Reuters/Landov

Air traffic connections from West Africa to the rest of the world: While Guinea, Liberia and Sierra Leone don't have many flights outside the region, Nigeria is well-connected to Europe and the U.S. PLOS Currents: Outbreaks hide caption

itoggle caption PLOS Currents: Outbreaks

A Public Health Agency of Canada worker seen inside the National Microbiology Laboratory's Level 4 lab in Winnipeg. Public Health Agency of Canada/Nature hide caption

itoggle caption Public Health Agency of Canada/Nature

A technician tests samples from Ebola-infected patients at a field lab, run by Doctors Without Borders, in Kailahun, Sierra Leone. Tommy Trenchard for NPR hide caption

itoggle caption Tommy Trenchard for NPR

A scientist tests a patient's blood for Ebola at the European Mobile Laboratory in Gueckedou, Guinea. The first cases reported in the outbreak occurred in a small village about eight miles outside Gueckedou. Misha Hussain /Reuters /Landov hide caption

itoggle caption Misha Hussain /Reuters /Landov

Jude Law prepares for the looming pandemic in the 2011 movie Contagion. There are huge differences between viruses in movies and Ebola in real life. Warner Bros/The Kobal Collection hide caption

itoggle caption Warner Bros/The Kobal Collection

Dr. Kent Brantly (center) announces his recovery from Ebola, with his wife, Amber Brantly (left), during a press conference at Emory University Hospital Thursday in Atlanta. Brantly got sick at the end of July. Jessica McGowan/Getty Images hide caption

itoggle caption Jessica McGowan/Getty Images

Dr. Bruce Ribner, medical director of Emory University Hospital's infectious disease unit, embraces Dr. Kent Brantly (left) who was treated with an experimental Ebola medicine and released from the Atlanta hospital Thursday. John Bazemore/AP hide caption

itoggle caption John Bazemore/AP

Workers with the aid group Doctors Without Borders prepare a new Ebola treatment center near Monrovia, Liberia, on Sunday. The facility has 120 beds, making it the largest Ebola isolation clinic in history. John Moore/Getty Images hide caption

itoggle caption John Moore/Getty Images

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