Nearly half a millennium ago science took a great leap forward with the discovery of the microscope. Before its existence it was postulated that "little creatures," too small to be seen by the naked eye, existed; however, it was not until the discovery of the microscope that this could be demonstrated.
Two men are credited today with the discovery of microorganisms using primitive microscopes: Robert Hooke who described the fruiting structures of molds in 1665 and Antoni van Leeuwenhoek who is credited with the discovery of bacteria in 1676.
Many years later, the emergence and progression of the discipline of microbiology was able to resolve two important conundrums that had prevailed in science: the existence of spontaneous generation and the nature of infectious disease.
Spontaneous generation of bacteria and other organisms was thought to be the driving process of putrefaction. This, however, was debunked by Louis Pasteur whose research on sterilization clearly indicated that this was not the case.
Robert Koch's research, famously dubbed "Koch's postulates," demonstrated that infectious disease was caused by microorganisms and therefore shed light on the nature of infectious disease.
The impact of the emergence of microbiology is monumental, not simply because of the scope of understanding that we have gained from its discovery, but also in terms of the increased prosperity of humans that has occurred as a result of our understanding of these "little creatures."
To put the latter statement in perspective: in the year 1900 the prevailing three causes of death were influenza/pneumonia, tuberculosis and gastroenteritis, whereas in the year 2000 the prevailing causes of death were heart disease, cancer and stroke. This represents a strikingly different etiology of deaths and it will be interesting to see how these trends continue to change.