A deleterious set of responses which occurs at the subcellular level, stimulated by some injury, and which is often manifested in altered structure or functioning of the affected organism. The concept of disease has been changing as understanding of biological systems has increased. For centuries disease was described in terms of gross changes. Then a major revolution in biology occurred between 1840 and 1860, based largely on the improvement and application of the light microscope. It included the works of Jacob Shleiden and Theodore Schwann, who developed the concept referred to as the cell theory. This theory states that the basic structural and functional unit of all plant and animal life is the cell. R. C. Virchow was the central figure who applied the cell theory to the pathogenesis of human disease. He recognized that diseases were the product of the alteration of cell structure and function.