The Romans were the ones who discovered that if you had a piece of glass that was thin on the edges but thick in the middle, it would make things look larger. Until the 13th century when they were used to make spectacles, these glasses were not much used except by people who were interested in studying insects -- so they were called "flea glasses." They were the earliest form of microscope, very similar to a magnifying glass today. They typically magnified their subject only by 6 to 10 times.
In 1590 Zacharias and Hans Janssen, spectacle makers, discovered that if you combined more than one lens together then their effect was amplified. They created the first compound microscope, which is a microscope with two or more lenses. Galileo heard of their experiments and added his own improvements, also building telescopes to study the stars. The first microscope to reveal the unseen to its inventor was made by Anthony Leeuwenhoek of Holland. He was able to create lenses that would magnify by 270X and saw bacteria, blood cells and more. He is called the Father of Microscopy.
Leeuwenhoek went on to write many papers about his discoveries. He described protozoans in pond water and bacteria from teeth scrapings. He depended in turn on the writings of 17th century scientist and inventor Robert Hooke, who used a compound microscope to discover that all living things are made of cells, and wrote a book called Micrographia on the subject. Since then one scientific discovery has followed another as microscopes have improved people's ability to look at the tiny and complex parts of the world.
Science and medicine always go hand in hand. As more was understood about how human bodies work, so understanding of disease and treatment increased. Louis Pasteur discovered yeast fungus with a compound microscope. By the same means, Hooke and Leewenhoek together were able to figure out how the human circulatory system works. People owe their current level of medical knowledge to the information gained through microscopic study.