Biologist Rob Knight, co-founder of the American Gut Project, recently moved the project to the University of California, San Diego's School of Medicine. Casey A. Cass/University of Colorado hide caption

itoggle caption Casey A. Cass/University of Colorado

Bruno Mbango Enyaka gets his flu shot at a community health center in Portland, Maine, on Jan. 7. Gabe Souza/Press Herald via Getty Images hide caption

itoggle caption Gabe Souza/Press Herald via Getty Images

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

Last fall's state-ordered quarantine of nurse Kaci Hickox (shown here with her boyfriend, Theodore Michael Wilbur, in late October) started at the airport in Newark, N.J., then followed her home to Fort Kent, Maine. Hickox treated Ebola patients in Africa but never had the illness. Spencer Platt/Getty Images hide caption

itoggle caption Spencer Platt/Getty Images

The hepatitis C medication Sovaldi, from Gilead Sciences, costs $1,000 per pill. It's just one of the new medications introduced in the past year that can cure the disease within weeks or months. Courtesy of Gilead Sciences via AP hide caption

itoggle caption Courtesy of Gilead Sciences via AP

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

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