They are many inventions which were made during medieval times or middle ages, all of which were outstanding, inventions like the tidal mill, hourglass, eyeglasses and even liquor. But the very invention which stands out amongst the rest for me is the microscope.
You know the saying "there's more than meets the eye" glorifies the microscope in the manner of it's use. I've always wondered how the health state of people from that time was. Their inability to understand certain biology in both organism and their ultrastructure. An example of such situation is undoubtedly the infamous black death.
Black death which was also know as the Great plaque was one of the most devastating pandemic in human history. It claimed the life of over 200 million people resulting to the deaths of 30 - 60% of Europe's population between 1347 and 1351. It took 200 years for the world to recover the lost population. This plaque was caused by bacterium yesina, something physicians knew nothing about.
The arrival of microscopes changed all that and amongst many of it's benefit, it led to the ground breaking discovery of many things like our red blood cells and micro organism. It was a huge strive for medical science. The microscope still does wonders till date. It's no wonder many people fought for the rights to the invention during it's early stages.
A little history about microscope: although the earliest known record for the use of magnifying glasses dates back to the 13th century, records of history has shown other glass object resembling lenses as far back as 4000 years ago. And although certain credits has been given to Roger Bacon who in 1268 was recorded to have used lenses for optical purposes, no one knows for sure who invented them as both the west and China were recorded to have used lenses inserted on frames as magnifying glasses for reading. Other several claims have been made by over a dozen people on the invention and even patent rights were been submitted to relevant bodies. The most widely recognized is that of Galileo who had called it "occhiolino or little eye" in 1624.