Viruses and Koch's Postulates1

Diseases at one time were thought to be caused by wrath of the gods, configuration of stars or miasmas. After a real struggle that occurred not so many years ago, certain maladies were shown to be induced either by small animals or minute plants, e.g., protozoa, fungi, bacteria and spirochetes. Indeed, the victory was so great that most workers in time began to consider that all infectious diseases, including those whose incitants had not been discovered, must be caused by agents similar to those already recognized. According to them, there could be no infections that were not caused by protozoa, fungi, bacteria or spirochetes, and to intimate that some infectious agents might be inanimate constituted heresy of the first order. Even at the present time, the cause of certain diseases is said by some individuals to be unknown or undiscovered, because no cultivable bacterium or visible protozoan parasite of etiological significance has been demonstrated in them. For instance, a few years ago Cowie made the statement in a scientific paper that the etiological agent of poliomyelitis is unknown, and in the recent book, An American Doctor's Odyssey, Heiser remarked that "the microbe which causes smallpox has never been discovered." In spite of the general acceptance of the idea that all infectious diseases are caused by protozoa, fungi, bacteria or spirochetes, some workers have always contended that there might exist other infectious agents incapable of classification with those already known. Furthermore, very early in the bacteriological era a few discerning individuals appreciated the fact that there was no reason, except analogy, for assuming that all infectious agents must be living autonomous organisms. Through the activities