No one will ever know. It is a medical myth that makes people believe medicine and science know the cause of disease or ailments.
Diagnoses and causes are nothing more than hypothesis also known as BEST GUESS.
Speculation is all you will get. Medicine can only speculate as to what caused the "mystery" illness the same as they can only speculate as to what causes high blood sugar often referred to as diabetes or what causes sores often referred to as herpes.
Because practicing medicine is nothing more than guessing it isn't any wonder why nothing get cured or prevented these days. That is why humans are devolving while all other life is evolving.