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Since the dawn of history, human beings have witnessed the appearance of epidemic or epizootic diseases. The suddenness and the prevalence of these plagues were generally considered to be connected with occult influences of the stars or planets upon human affairs, climatic changes or religious reasons. Slowly, the principle of the origins of contagious diseases has become better understood and the role of transmissible influences such as parasites, bacteria and viruses has been accepted. A landmark was discovery of the germ theory, which included small parasites, bacteria and viruses. This theory was mainly based on the studies of Koch, Lister, Pasteur and many others.