With the invention of the microscope, humanity became aware of the microworld which is defined as 1 to 1000 microns. A micron is equal to one thousands of a millimeter. A cell is about 10 microns wide. Paper is about 100 microns thick. The unaided human eye can see items as small as 50 microns, or about half the width of a piece of paper.

Scientists use three scales when talking about the biological world: the milliworld, microworld, and the nanoworld. The milliworld contains all visible items down to 1 millimeter and includes very small things such as ants, fleas, and grains of sand.

By 1640, the microscope was perfected to the point that allowed the introduction of the microworld to humanity. The microworld contains items with a diameter from 1 millimeter to 1 micrometer, or 1 micron. The microworld contains things like single celled organisms as well as the largest bacteria. By 1640, humanity started it’s introduction to trillions of organisms living everywhere including nearly everything you touch, in the ground, inside plants, and even inside humans. Humans host over 10,000 species of organisms in, on, and through the human body — known as the human microbiome. Every human no matter how clean you think you are is playing host to 10-100 trillion organisms. This fact gradually changed how every human on Earth views life.

The nanoworld which includes smaller bacteria as well as viruses, proteins, and molecules would have to wait for the invention of the electron microscope in 1931. 

Note: The micron and micrometer are the same size, but you use microns (μ) when measuring thickness, and micrometers (μm) when measuring the distance between things. So you can say a cell is 10 microns wide, or you can say the diameter of a cell is 10 micrometers. The term nanomicron, which would be equal to a nanometer, is not currently in regular use.