• The Italian scientist Fracastorius (1485-1553), understood the concept of disease and postulated that the disease is transmitted by individual particles or seeds from one person to another.
• Anton van Leeuwenhoek (1685), with his microscope, that was little over 2 inches, described the existence of microbes. In his letter to the Royal Society of London, September 17, 1683, described may little life forms those can't be seen with out a microscope.  

Although people understood about the existence of microbes, but could not be related to the diseases, till 1857.

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Further Reading on the Microbes Time Line Please read:

http://www.mansfield.ohio-state.edu/~sabedon/biol2007.htm
http://www.cdc.gov/drugresistance/community/faqs.htm