Infectious diseases were familiar in early periods and it was considered that they spread by contact, even though no one knew exactly how this happened. Epidemics occurred and frequently spread - the Black Death in 14th century Europe is a well-known example - and this perpetuated the widespread belief in many societies that they were due to paranormal causes. Undeniably epidemics were often considered to be divine punishments. The first definite suggestion of a scientific cause came from Hierinymous Fracastorol who thought that diseases were transmitted by "seeds of infection" either by contact, indirect contact or by transmission from a distance. These 'seeds', were thought to have faculty of multiplying and propagating rapidly.
Long before, in the hazy unrecorded past, someone picked up a piece of transparent crystal thicker in the middle than at the edges, looked through it, and discovered that it made things appear larger. Someone also found that such a crystal would focus the suns rays and set fire to a piece of parchment or cloth.
Magnifiers and "burning glasses" are mentioned in the first century A.D., but apparently they were not used until the invention of spectacles, toward the end of the 13th Century. They were named lenses because they are shaped like the seed of a lentil.
The Dutch scientist Antoni van Leeuwenhoek around 1660 invented the earliest microscope. It contained a single round glass lens about the size of a raindrop. Van Leeuwenhoek realized that a glass sphere has a maximum possible curvature. As a result, a spherical lens has the maximum magnification of about 300 times. The invention of this microscope opened a whole new world. For the first time, people could see pollen grains from flowers, bacteria and sperm from male animals.
Although millions of microscopic examinations have confirmed this observation, it...