Mass hysteria (also known as epidemic hysteria) is a condition in which a large group of people exhibit the same inexplicable physical or emotional symptoms. The most famous example of mass hysteria is the Salem Witch Trials, where people were so affected by paranoia and prosecution that they began accusing each other of witchcraft. I believe they acted out in self-preservation, to an extent they truely believed they were being affected by witchcraft.
When it comes to preventing mass hysteria, we first have to look at the causes. In most cases, mass hysteria erupts from heightened anxiety. For example, when there were news of an outbreak, people were more likely to panic over that than the flu - which kills every year by the thousands. This may be caused by how the news is delivered, especially since public panic guarantees more views.
While it may be difficult to prevent something as contagious as mass hysteria from spreading, the best defense is to be informed of mass hysteria itself and the situation at hand. The more people are aware of it, and address it with critical thinking, the closer we are to its prevention.