The Germ Theory, which emerged in the late 19th century, demonstrated that microscopic germs caused most human infectious diseases. The germs involved included bacteria, viruses, fungi, protozoa, and prions. Louis Pasteur (1822-1895), a French chemist and microbiologist, and Robert Koch (1843-1910), a German physician and microbiologist, are credited with the discovery of the germ theory in the...