Jim Nepstad, superintendent of Effigy Mounds National Monument in northeast Iowa, stands at the top of a bluff looking over the Mississippi River. Clay Masters/NPR hide caption

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In this frame grab taken from video provided by the New York Police Department, an unidentified man approaches New York City police officers with a hatchet, on Thursday. The man was fatally shot by police after he wounded two officers. AP hide caption

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Dontey Carter (from left), Mel Moffitt, Lenard Smith, Ned Alexander and Allen Frazier are all members of the Lost Voices group, formed after Michael Brown's death in August. They say they want to ensure justice for Michael Brown and other unarmed individuals killed by police officers. Cheryl Corley/NPR hide caption

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Police officers stand outside the home of Craig Spencer, a Doctors Without Borders physician who recently returned to the city after treating Ebola patients in West Africa, on Thursday in New York. Spencer tested positive for the virus, according to preliminary test results, city officials said. John Minchillo/AP hide caption

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A makeshift memorial sits on a downtown street a block away from Canada's National War Memorial in Ottawa to remember Canadian soldier Nathan Cirillo, who was shot and killed by an assailant Wednesday. Warren Toda/EPA/Landov hide caption

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Frank Mankiewicz, Sen. Robert F. Kennedy's press secretary, updates the press about Kennedy's condition after being shot in June 1968. Mankiewicz died Thursday at the age of 90. AP hide caption

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At the onset of symptoms, Dr. Adaora Ingonoh (center) and her colleagues began drinking oral rehydration solution. It doesn't taste great but they say it helped them survive Ebola. They each downed over a gallon a day for nearly a week. Andrew Esiebo/Courtesy of WHO hide caption

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The phone company Vonage reported a drop in voice mail retrievals over the past year. Many of those ignoring voice mails are millennials. iStockphoto hide caption

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Data sources: David Ropeik/Harvard University, National Weather Service, World Health Organization, Northeastern University Laboratory for the Modeling of Biological and Socio-Technical Systems, National Geographic, United States Census Adam Cole and Ryan Kellman/NPR hide caption

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