Even before microorganisms were seen, some investigators suspected their existence and responsibility for disease. Among others, the Roman philosopher Lucretius (about 98–55 B.C.) and the physician Girolamo Fracastoro (1478–1553) suggested that disease was caused by invisible living creatures. The earliest
microscopic observations appear to have been made between 1625 and 1630 on bees and weevils by the Italian Francesco Stelluti, using a microscope probably supplied by Galileo.However, the first person to observe and describe microorganisms
accurately was the amateur microscopist Antony van
Leeuwenhoek (1632–1723) of Delft, Holland (figure 1.1a).
Leeuwenhoek earned his living as a draper and haberdasher (a
dealer in men’s clothing and accessories), but spent much of his
spare time constructing simple microscopes composed of double
convex glass lenses held between two silver plates (figure
1.1b). His microscopes could magnify around 50 to 300 times,
and he may have illuminated his liquid specimens by placing
them between two pieces of glass and shining light on them at a
45° angle to the specimen plane. This would have provided a
form of dark-field illumination (see chapter 2) and made bacteria
clearly visible (figure 1.1c). Beginning in 1673 Leeuwenhoek
sent detailed letters describing his discoveries to the Royal
Society of London. It is clear from his descriptions that he saw
both bacteria and protozoa.