A teen gets a dose of Pfizer's COVID-19 vaccine last month at Holtz Children's Hospital in Miami. Nearly 7 million U.S. teens and preteens (ages 12 through 17) have received at least one dose of a COVID-19 vaccine so far, the CDC says. Eva Marie Uzcategui/Bloomberg via Getty Images hide caption

Scientists once compared the abilities of humans versus canines in tracking a trail of chocolate essential oil laid down in an open field. Though the humans weren't nearly as proficient as the dogs, they did get better with practice. Vladimir Godnik/Getty Images/fStop hide caption

Southern Californians celebrate at a mass vaccination site in Disneyland's parking lot in January. CDC head Dr. Rochelle Walensky cautions that for strongest immunity, recipients get both doses of the Pfizer or of the Moderna vaccine according to schedule. Mario Tama/Getty Images hide caption

Staff and residents of the Ararat Nursing Facility in the Mission Hills neighborhood of Los Angeles got COVID-19 shots on Jan. 7. Coronavirus cases, hospitalizations and deaths have been surging throughout Los Angeles County. Mario Tama/Getty Images hide caption

Pfizer-BioNTech's COVID-19 vaccine has been tested for safety and efficacy in more than 44,000 people. Still, stopping viral spread will take more than immunizations, says the CDC. The agency is calling for those who are vaccinated to continue wearing masks and practicing safe physical distancing. Frank Augstein/AP hide caption

A new study from the New England Journal of Medicine finds that motor vehicle crashes were one of the leading causes of death among children and adolescents in the U.S. in 2016. Tim Graham/Getty Images hide caption

A migrant receives medical attention at a former paper factory in Greece that has been turned into a makeshift camp. Menelaos Michalatos/SOPA Images/LightRocket via Getty Images hide caption

The elaborate Alnwick Garden in northeast England includes a "Poison Garden" that showcases plants with killer properties. Visitors are invited to look but not touch or even smell. Joanne Silberner for NPR hide caption

High school students in Tanzania gather in a Mental Health Listening Club — first comes the soap opera, then the chance to ask questions about topics like depression. Omar Dabaghi-Pacheco for Farm Radio International hide caption

A mentally ill patient in Afghanistan at the Mia Ali Baba holy shrine in the village of Samar Khel. The mentally ill are thought to be possessed by demons and so are chained for 40 days. Noorullah Shirzada/AFP/Getty Images hide caption

Untreated depression can harm the health of mothers and their children. iStockphoto hide caption

A health worker injects a vaccine to prevent polio and measles in Lahore, Pakistan. K.M. Chaudary/AP hide caption

Physicians Nowiba Mugambi and Erica Palys discuss a patient's X-ray at the Moi Teaching and Referral Hospital in Eldoret, Kenya. The hospital plans to open a new cancer treatment center in April. Evelyn Hockstein/Courtesy of AMPATH hide caption

Tony McMichael has written more than 300 papers on how erratic weather and climate can cause health problems. He died in September. James Giggacher/Courtesy of Australian National University hide caption

The documentary film, "Every Last Child," chronicles the efforts to eradicate polio in Pakistan. Women play a key role — they're welcome in homes to share information, while men are not. Courtesy of "Every Last Child" hide caption

A look at some of the stuff I snagged in the exhibit hall of the American Public Health Association meeting in Denver. Joanne Silberner/NPR hide caption