A mysterious disease swept the world in the early 1920s that put people into catatonic states; stiff and immovable like human statues. It was called encephalitis lethargica and it affected over 5 million people worldwide, and then was almost forgotten.
Knowing what we know now, what do you suppose caused this?
I have a personal theory that stronger microscopes allowing us to see live micro-organisms in live tissue that may not even be known of yet will be the key to understanding many illnesses.