Discovery of the Microorganisms within the Dental Plaque
by Anton van Leeuwenhoek (1632-1723)
  Excerpt from one of his letters sent to royal Society of London:

“Tho my teeth are kept usually very clean, nevertheless when I view them in a Magnifying glass, I find growing between them a little white matter as thick as wetted flower… I therefore took some of this flower and mixt it … with pure rain water whererein were no Animals … and then to my great surprize perceived that the aforesaid matter contained very small living Animals, which moved themselves very extravagantly … The number of these Animals in the scurf of a mans Teeth, are so many that I believe they exceed the number of Men in a Kingdom. For upon the examination of a small parcel of it, no thicker than a Horse-hair, I found too many living Animals therein, that I guess there might have been 1000 in a quantity of matter no bigger than the 1/100 part of a sand.”