John Poole

Planet Earth is a vast place, with humans scattered all over it.

For most of human history, we had a lot of bad ideas about how we were getting sick. We also had plenty of bad ideas about how to prevent it, like bloodletting, drilling large holes in the head and drinking arsenic.

We really only started to figure out how to effectively fight infectious disease about 200 years ago, when, inspired by milkmaids, a doctor named Edward Jenner decided to take a closer look at a promising folk remedy - the surprising details we'll leave for the video.

Ten thousand years ago, at the dawn of the agricultural revolution, many of our worst infectious diseases didn't exist.

Here's what changed.

Humans get along pretty well with most microbes. Which is lucky, because there are a lot more of them in the world than there are of us. We couldn't even live without many of them. But a few hundred have evolved, and are still evolving, to exploit our bodies in ways that can make us really sick. These are the microbes we call germs. Think plague, flu, HIV, SARS, Ebola, Zika, measles.

This is a series is about where germs come from. In this first of three episodes, we see what our early encounters with germs may have been like — and how germs first got the upper hand.

The world's girls are healthier than ever. They live longer and more of them are going to school than at any time in history.

But most of them face discrimination simply because they are girls. The discrimination happens at every point in their lives.

In some cases, it starts even before they're born, when parents decide to abort a pregnancy if the fetus is female.

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