We are never told the truth about such things, simply left to obey or disobey, to believe or disbelieve. I believe there was a virus travelling about, made much, much worse by a handful of bad actors intent on reducing population and putting a chokehold on those remaining unbridled.
The stupidity I saw first hand told just how much of the populace believed the end was near. Driving alone in your car? Better wear a mask. Sitting down to eat in a diner? Better wear a mask in between bites. Shopping in the supermarket in one way aisles? Yep, you're an idiot. Need a dentist appointment? Sorry, Charlie...no can do.
I lived life as I always have, going up and down grocery aisles not paying attention to the taped signs on the floor, which incidently cause a few lawsuits over tripping on those taped signs. I did wear a mask as I worked on the Space Force Base shucking eyeglasses and had to wear it or lose my job. I did wind up leaving there after a few weeks of that.
All in all, a very telling display of human fear and submission to threats fabricated by 'experts in the field'.