In 1858, at the Pathological Institute in Berlin, Virchow published Die Cellularpathologie. His argument was that disease does not reside in organs or tissues in general, but originates in individual cells, and that the nature of a disease can be understood by looking at those cells under a microscope.
He also developed the first systematic method of autopsy, and the modern autopsy still uses his technique.
He is called the father of modern pathology. Pathology is diagnosis pushed as far down as it will go.