Two kinds of evidence are recognized in the courts of law, direct and circumstantial. The first is positive and, if true, it is incontrovertible. Circumstantial evidence, however, though it may be convincing, still deals only with possibilities and probabilities and may be refuted by direct evidence or by stronger circumstantial facts.
Practically speaking, all studies in the etiology of disease are based on purely circumstantial evidence. We sometimes become so accustomed to the deductions of certain observations that we lose sight of the possibility of error. Koch's postulates are, perhaps, as clear and convincing as any scientific reasoning can be: A germ isolated from the blood or body of a person sick or dead from a given disease; that germ cultivated outside the body until a pure culture is obtained; that pure culture being introduced into a healthy individual, and such inoculation being followed by the typical symptoms of the
HEMENWAY HB. THE SCARLET FEVER EPIDEMIC OF NINETEEN HUNDRED AND SEVEN.. JAMA. 1908;L(14):1115–1121. doi:10.1001/jama.1908.25310400027003
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