Okay, first off I want to just preface this by saying I'm not really that well versed in history at all, so I apologize if I mess any facts up.

From what I understand, ancient Greeks and Romans were much more advanced than the majority of Europe in the middle ages. Not only did they have a working sewage system through the use of aqueducts, but they also developed a number of pioneering medical practices, and were able to engineer complex machines such as the Antikythera mechanism. But then 800 years later, all of a sudden people are just living in their own filth and bleeding themselves to get rid of headaches.

What happened to cause such a regression?