The Middle Ages or as some may call it the Dark Ages and for good reason, because this was the fall of the mighty Roman Empire. What this did was stop the rate of advancements that the Romans were doing especially in technology.
Something that did arise from the Dark Ages is the mass increase in the amount of religious beliefs, particularly Christians. People would spend a lot of time in churches reading the Bible and praying to their God(s) which left hardly anything to be recorded hence why it is called the Dark Ages because there was no evidence that anything amazing happened.
As a mass amount of the people were going to Churches the people who owned the Churches became very wealthy, so there was still a lower and higher class in Dark Ages, another thing which let the increased rate of intelligence and evolvement slow down dramatically was that there was a lack of education as everyone was only being educated by the Bible.
So what did advance, well the same thing that has been increasing since Cavemen, art and culture the churches were decorated beautifully and the arts in general improved dramatically.
Then we get to the terrible Black Death caused by the plague which was from the world being ridiculously unhygenic causing a virus which caused death. This crossed throughout the world and it is said over 33 MILLION people died in Europe and 100 MILLION worldwide, an estimate of 30-60% of the worlds population gone.
So what caused the plague to stop, well the amount of people to spread it was decreased from the amount of deaths, but it is said that the great London fire caused all the virus in that area to be vaccinated. How? Well the fleas and other rodents were a massive cause of diseases and as the fire exterminated a huge amount of the Rats and Fleas died. Also not to mention the fire caused all of the waste in the streets to be burnt. It was like a new beginning a second chance for humanity.