Hey kids, if you've ever been Black Friday shopping or visited the Diablo subreddit recently?

Chances are you've encountered mass hysteria at some point.

Mass hysteria is known medically as mass psychogenic illness, or MPI for short.

It's basically just when a bunch of people start acting like fool for no discernible reason other than maybe a stressful environment.

Being that this sort of thing is naturally very noticeable, there's loads of documented cases found all throughout history.

Let's take a look at a few.

This first one is what made me make this video in the first place.

So one day sometime in the Middle Ages a group of nuns in a French convent were enjoying a quiet uneventful day, Until one of them decided to start meowing.

You know, like a cat.

You'd think this would last all of four seconds before another nun was like, "excuse me, sister Gertrude. Would you kindly cut the sh*?" But instead, another nun joined in, and another.