For the first time in history, humans were living in close contact with domesticated animals - milking them, taking care of them, living with them and eating them. All that touching and sharing gave animal germs plenty of chances to get inside of 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. Now that humans were spending so much quality time with cows, little Rinderpest germs started jumping over into us. And a few of them had a lucky mutation that allowed Rinderpest to evolve from a cattle disease into measles, a deadly human virus.
But our ancestors had no idea what the problem was - or how to fix it.