A groundskeeper at Pinecrest Gardens sprays pesticide to kill mosquitoes in Miami-Dade County, Fla., in 2016. Gaston De Cardenas/Miami Herald/TNS via Getty Images hide caption

Hospital-acquired infections are a big risk in health care, especially for older or seriously ill patients. Dana Neely/Getty Images hide caption

Dr. Thumbi Mwangi, an infectious disease epidemiologist from Kenya, at Howard Theatre in Washington, DC, on Nov. 29, 2016. In the U.S., Mwangi worked on a vaccine for cows that aimed to combat the same disease he saw the bovine battle in Kenya as a kid. Akash Ghai/for NPR hide caption

Fishermen in Papua New Guinea, living on their boats, wait for the tide to change before going out to fish. Tuberculosis is a major health threat in the Pacific Ocean nation. Jason South/The AGE/Fairfax Media via Getty Images hide caption

Researchers used online data to model the vaccination rate in communities affected by an outbreak of mumps in Arkansas. Maimuna Majumder/HealthMap; Alyson Hurt/NPR hide caption

Tucker Lane and his mother, Lynn Cash, sit in the wooded backyard of his home in West Barnstable, Mass. Kayana Szymczak for NPR hide caption

When a single raindrop hits the ground, it causes tiny water droplets to shoot into the air, some containing bacteria. Joung et al. Nature Communications hide caption

Pig farm workers push live pigs into a large grave in Nipah in 1999. To stop the outbreak, the Malaysian government culled almost 1 million pigs, nearly destroying the country's pork industry. Andy Wong/AP hide caption

Andie Vaught grasps a stress toy in the shape of a truck as she prepares to have blood drawn as part of a clinical trial for a Zika vaccine at the National Institutes of Health in Bethesda, Md., in November 2016. Allison Shelley/The Washington Post/Getty Images hide caption

The Golden Age Of Germs Xaver Xylophon/for NPR hide caption

Heat and steam from your shower or shave can rob medicine of its potency long before the drug's expiration date. Angela Cappetta/Getty Images hide caption

Under the old rules, the CDC's authority was primarily limited to detaining travelers entering the U.S. or crossing state lines. With the new rules, the CDC would be able to detain people anywhere in the country, without getting approval from state and local officials. Mark Wilson/Getty Images hide caption

A Short History Of Humans And Germs Xaver Xylophon for NPR hide caption

Truvada, when taken daily, can vastly reduce the risk of getting HIV in people at high risk. Jeff Chiu/AP hide caption