This disastrous epidemic was caused totally by extraterrestrial interference with living humans to alter the mind and cause the symptoms of becoming disconnected from the ability to produce willful action of the body, including speech and movement, thus trapping the mind in a body that is frozen in the moment and quite limited in what it can do. It is akin to being entombed, buried alive, but still possessing senses and knowing stimuli are being received, but being unable to respond in any way actively. So it is not only incapacitating but deeply traumatic to the being who must endure this, often for a lifelong experience.
In this way, it is particularly diabolical. It has tapered off because the extraterrestrials have simply moved on to other mischief and realizing they must be a bit more clever to not do something too far outside known laws of physics because the scientific community would be raising too many questions. It is safer to use microorganisms and modify them if need be to have useful vectors of transmission and an enhanced virulence, if that is a desired outcome, because this will all seem to be natural, if unexpected, based on past history. But humans are used to having new infectious diseases emerge and it is typically assumed it has been in the environment all along, but heretofore unrecognized, and once discovered, it does not seem unusual.