The bubonic plague, or the black death.
Started in 1347, came from Asia via Italian merchants. (Why didn't the Asians get it? they had immunity because they won the challenge on last week's episode)
Was a bacteria that was carried by rats. The rats also carried fleas (they were equal opportunity pests). The fleas bit the rats and got the bacteria. Then they bit the nasty, non-bathing, unhygienic middle-agers and presto infecto.
Once you came down with the initial symptoms (headache, fever, vomiting, weakness) you really only had about 3 days left. The for-sure sign that you had it was the buboes, or swellings that would appear in the groin and armpits.
The plague swept through Europe between 1347 and 1350. By the time it had run it's course, 1/3 of the population had died. People thought it was the end of the world. The plague completely rocked the social structure of medieval Europe. The deeply entrenched system of feudalism could no longer be supported. There just weren't enough people.
The plague wound it's way around Europe for the next few hundred year, but it seems it was never quite as bad as the initial outbreak.
In case you were wondering, yes, it was hard to study a topic like this with small children. They were very worried about getting sick. They were very worried that we had rats in our house. I had to explain over and over that we have antibiotics and that we are hygienic (we give our trash to the garbage man, we don't throw it in the streets).
But we still had to do a project or craft. That's what we do on Fridays to make school fun and memorable. But how do you put the fun in Black Death?