One form of carbapenem-resistant bacteria seen in culture. CRE bacteria are blamed for 600 deaths a year in the U.S. and can withstand treatment by virtually every kind of antibiotic. CDC hide caption

itoggle caption CDC

Measles is highly contagious, and it produces fever and rash in susceptible people who become infected. Hazel Appleton/Health Protection Agency Centre/Science Source hide caption

itoggle caption Hazel Appleton/Health Protection Agency Centre/Science Source

Dr. Kwan Kew Lai volunteered for six weeks at an Ebola treatment center run by International Medical Corps in Bong, Liberia. Courtesy of Kwan Kew Lai hide caption

itoggle caption Courtesy of Kwan Kew Lai

The Pseudomonas stutzeri bacterium, commonly found in soil, was the most prevalent subway microbe. Lower Manhattan was its prime hangout. Mason/Cell Systems 2015 hide caption

itoggle caption Mason/Cell Systems 2015

Leah Russin, of Palo Alto, Calif., holds her son, Leo, 16 months, as she speaks Wednesday at a news conference in support of proposed state legislation that would require parents to vaccinate all school children. Rich Pedroncelli/AP hide caption

itoggle caption Rich Pedroncelli/AP

Jackie Carnegie immunizes Mabel Haywood in a Colorado Health Department immunization van in 1972. Shots for measles and other infectious diseases were offered. Ira Gay Sealy/Denver Post Archive/Getty Images hide caption

itoggle caption Ira Gay Sealy/Denver Post Archive/Getty Images

Writer Roald Dahl and his wife, actress Patricia Neal, with two of their children, Theo and Chantel Sophia "Tessa." The photo was taken a few years after oldest daughter, Olivia, died of measles. Hulton Archive/Getty Images hide caption

itoggle caption Hulton Archive/Getty Images

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