Harvard University Library: "Robert Koch, 1843 - 1910"
Read this page. Note the significance and limits of Koch's postulates in the identification of the causative agent. Koch took the first photomicrograph of bacteria and the first photomicrograph of bacteria in diseased tissue.
Click http://web.archive.org/web/20160529035220/http://ocp.hul.harvard.edu/contagion/koch.html link to open resource.