One of the worst medical disasters in European History was the Bubonic Plague, also known as the Black Death. It started in the Far East and traveled westward, killing millions on the way. It reached Europe in 1347 and killed one third of the population in its initial bout.
It has been taught in schools for decades, but no one truly knows its origins. It was a failed experiment by a nameless organization. Their goal? Create a pathogen that increases the abilities of the immune system. What they created instead? Something that attacks the immune system.
No one knows about this because in the "official" historical records, the technology to create super pathogens did not even exist then. In truth, a lot of secret organizations were ahead of their times.