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It’s not easy to say that any particular scientific development was the most important in a given year. But if we had to choose some highlights, we’d opt for these unforgettable events and findings. (12/20)

Ever since scientists created the powerful gene-editing technique Crispr, they have braced for the day when it would be used to produce a genetically altered human being. Now, the moment they feared may have come. What’s likely to happen next? (Barbaro, 12/19)

On a sunny autumn afternoon, Jayden Castillo and Janelle Moore sit at a suburban Wendy’s, sipping strawberry lemonades and talking about the realities of sex for American teens. The old friends, both seniors at Oxon Hill High School in Maryland, believe access to porn has warped their generation’s concept of sex — especially in the minds of boys. “They learn everything from online,” Janelle explains. (McCarthy, 12/19)

You don't need to hang the mistletoe higher but you might want to skip the holiday cookies. A new report released Thursday shows U.S. adults aren't getting any taller but they are still getting fatter. The average U.S. adult is overweight and just a few pounds from obese, thanks to average weight increases in all groups — but particularly whites and Hispanics. (Stobbe, 12/20)

The AP found that certain demographic qualities — high rates of unemployment, low household income, a concentration of black or Native American residents and low rates of high school education — affected life expectancy in most neighborhoods. Their findings support the idea that many people experience bigger obstacles to good health because of their race or ethnic group, gender, age, geographic location and other social factors. (Washington, 12/19)

The 46-year-old man showed up at hospital emergency room in Chicago, complaining that his left knee hurt. The X-rays above showed why. Fourteen years earlier, he’d been shot in the knee. That’s the image on the left. The bullet was not removed, and over the years his knee had ground it into tiny pieces, as shown on the right. (Kolata, 12/19)

Children who live with cats and dogs when they are infants are less likely to develop allergies later in childhood — and the more pets they have, the better, a Swedish study of 1,278 children has found. Researchers interviewed the parents of 249 of the children when they were 6 to 12 months old, gathering information on pet ownership, and had clinical evaluations done at 18 months, 3 years and 8 to 9 years. (Bakalar, 12/19)

Kaiser Health News: When Needs Arise, These Older Women Have One Another’s Backs 

Like many women aging alone, Eileen Kobrin worried that an accident could compromise her independence. Then, two years ago at age 71, the New Yorker fell while on vacation, breaking her left ankle, and her Caring Collaborative network sprang into action. One member recommended an ankle surgeon at the nearby Hospital for Special Surgery, who operated successfully. Others brought over a wheelchair, a bath chair and an elevated toilet seat after Kobrin returned to her apartment with instructions to stay off her feet for several months. (Graham, 12/20)

Growers, scientists, epidemiologists, farm researchers and industry analysts know romaine lettuce sooner or later will carry E. coli onto America's salad plates. They just don't know why. Romaine has proven to be a reliable vehicle for the foodborne illness, and percentages dictate more outbreaks likely will occur. (Anglen, 12/19)