Michaeleen Doucleff

Six months ago, Melissa Nichols brought her baby girl, Arlo, home from the hospital. And she immediately had a secret.

"I just felt guilty and like I didn't want to tell anyone," says Nichols, who lives in San Francisco. "It feels like you're a bad mom. The mom guilt starts early, I guess."

Across town, first-time mom Candyce Hubbell has the same secret — and she hides it from her pediatrician. "I don't really want to be lectured," she says. "I know what her stance will be on it."

Last summer, Zac Peterson was on the adventure of a lifetime.

The 25-year-old teacher was helping archaeologists excavate an 800-year-old log cabin, high above the Arctic Circle on the northern coast of Alaska.

They had pitched tents right on the beach. Over the course of a month, Peterson watched a gigantic pod of beluga whales swim along the beach, came face-to-face with a hungry polar bear invading their campsite and helped dig out the skull of a rare type of polar bear.

But the most memorable thing happened right at the end of the trip.

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ROBERT SIEGEL, HOST:

There's a race on. The way to win is to eradicate a human disease. That's only been done once before - smallpox. This year, two diseases got tantalizingly close, but unexpected roadblocks have popped up. NPR's Michaeleen Doucleff reports.

For decades, scientists have been making predictions about how climate change will hurt health around the world.

But actually showing a link? That's been pretty tough.

Take for example, mosquito-borne diseases. It's easy to blame rising temperatures for the global spread of Zika or the explosion of dengue fever. Mosquitoes thrive in higher temperatures, right?

The words "endangered species" often conjure up images of big exotic creatures. Think elephants, leopards and polar bears.

But there's another of type of extinction that may be occurring, right now, inside our bodies.

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