After the earthquake in 2010, about 1,000 people were living in tents on the median of Highway 2, one of Haiti's busiest roads. Five years later, tens of thousands of people in Port-au-Prince still live in tents and other temporary housing. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

Ecologists found signs of Ebola in a Rousettus leschenaultii fruit bat. These bats are widespread across south Asia, from India to China. Kevin Olival/EcoHealth Alliance hide caption

itoggle caption Kevin Olival/EcoHealth Alliance

A woman protects her child's face in Managua, Nicaragua, as health workers fumigate for mosquitoes that carry chikungunya. The virus started spreading through Nicaragua and Mexico in the fall. Esteban Felix/AP hide caption

itoggle caption Esteban Felix/AP

The dengue virus has an icosahedral shape, similar to the pattern on a soccer ball. Antibodies stop the virus by binding to its surface. Laguna Design/Science Source hide caption

itoggle caption Laguna Design/Science Source

Members of the community in New Georgia Signboard greet President Ellen Johnson-Sirleaf Monday for the launch of the Ebola Must Go! campaign. John W. Poole/NPR hide caption

itoggle caption John W. Poole/NPR

A worker puts the finishing touches on the dividers that will separate patients at the community care center in the Port Loko district of Sierra Leone. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

Makutu Jabateh hugs her daughter, Mabana Konneh, 5, as the little girl returns home to her neighborhood in Jacobstown, Monrovia. John W. Poole/NPR hide caption

itoggle caption John W. Poole/NPR

Viruses can spread through the air in two ways: inside large droplets that fall quickly to the ground (red), or inside tiny droplets that float in the air (gray). In the first route, called droplet transmission, the virus can spread only about 3 to 6 feet from an infected person. In the second route, called airborne transmission, the virus can travel 30 feet or more. Adam Cole/NPR hide caption

itoggle caption Adam Cole/NPR

Dr. Komba Songu M'Briwah, left, talks on the phone while staff members disinfect offices at the Hastings Ebola Treatment Center in Freetown. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

India has record no Ebola cases, but the country is on high alert and has quarantined hundreds of travelers from West Africa. This hospital in New Delhi has set up an Intensive Care Unit for potential Ebola patients. Sajjad Hussain/AFP/Getty Images hide caption

itoggle caption Sajjad Hussain/AFP/Getty Images

Women who underwent sterilization surgery at a government-run camp were hospitalized in the Indian state of Chhattisgarh after 13 patients died following the procedure. Anindito Mukherjee/Reuters/Landov hide caption

itoggle caption Anindito Mukherjee/Reuters/Landov

Facebook CEO Mark Zuckerberg has launched an Ebola donation campaign on the website and tossed in $25 million of his money to help fight the virus. Ben Margot/AP hide caption

itoggle caption Ben Margot/AP

Private Ernest Cable was buried in a cemetery in Wimereux, France. He died from dysentery in a hotel turned hospital in the northern French town. Courtesy of Genome Research Ltd hide caption

itoggle caption Courtesy of Genome Research Ltd

David Quammen's new book is an extended excerpt of his previous one, Spillover, which explored how dangerous pathogens jump from animals to people. Courtesy of David Quammen hide caption

itoggle caption Courtesy of David Quammen