It is a phenomenon that transmits collective delusions of threats, whether real or imaginary, through a population in society as a result of rumors and fear.
In sociology and psychology, mass hysteria is also known as collective hysteria, group hysteria or, in laymen's terms, a big bunch of whining, juvenile crybabies.
They’re definitely disconnected from reality. Had a friend recently told me about a guy he knows who vehemently hates Trump. (Is there any other way?) Said the guy was bitching about his wife’s insurance premiums being so high with a huge deductable. My friend said “Didn’t she have great insurance before Obamacare.”. He said the guy just gave him a blank stare.