Ten thousand years ago, at the dawn of the agricultural revolution, many of our worst infectious diseases didn't exist.
Here's what changed.
With the rise of agriculture, for the first time in history humans were living in close contact with domesticated animals — milking them, taking care of them and, of course, eating them. All that touching and sharing gave animal germs plenty of chances to get inside us.
Take measles. Researchers think that up until about 5,000 years ago, it didn't exist. But its older cousin rinderpest, a cattle disease, did. When humans began spending so much quality time with cows, little rinderpest germs started jumping over into us. And a few of the germs had a mutation that allowed rinderpest to evolve from a cattle disease into measles, a deadly human virus.
As if that weren't bad enough, something else was happening around this time that supercharged the degree of damage this new measles virus could do. It has to do with the magic number of 500,000. When the world's first cities hit the half-million mark, it meant that there were now enough humans living together that measles and other germs had a steady and potentially endless supply of humans to infect.
Along with measles, scientists think other nasty diseases such as mumps, diphtheria, scarlet fever and whooping cough all evolved to live permanently in humans around 3,000 B.C.
But our ancestors had no idea what the problem was — or how to fix it. See how humans finally get a clue, in Episode 3, coming Feb. 16.
If you missed Episode 1, "Early Encounters," here's your chance to catch up:
What do you want to know about pandemics? Share your questions by submitting them in our special tool here. Our global health team will answer some of them in an upcoming story.