They happen fairly often. The cool thing about mass hysterias is that you don't know when you are in one. But sometimes the people who are not experiencing the mass hysteria can recognize when others are experiencing one, if they know what to look for.
I'll teach you what to look for.
A mass hysteria happens when the public gets a wrong idea about something that has strong emotional content and it triggers cognitive dissonance that is often supported by confirmation bias. In other words, people spontaneously hallucinate a whole new (and usually crazy-sounding) reality and believe they see plenty of evidence for it. The Salem Witch Trials are the best-known example of mass hysteria. The McMartin Pre-School case and the Tulip Bulb hysteria are others. The dotcom bubble probably qualifies. We might soon learn that the Russian Collusion story was mass hysteria in hindsight. The curious lack of solid evidence for Russian collusion is a red flag. But we'll see how that plays out.