Robert Koch isolated Vibrio cholera and Mycobacterium
tuberculosis using not only sound microbiological techniques
but also rigorous epidemiological formulations. Koch noted
that a specific microorganism can be identified with disease if
it is present in all cases of disease and absent in health, if it
can be isolated from diseased animal and grown in pure
culture, and if the fresh microorganism, when inoculated into a healthy laboratory animal, caused the same disease seen in
the original animal and could be reisolated in pure culture
from the experimental infection.Use of these postulates,
however, was deemed problematic for chronic diseases or
exposures that may have predated disease onset by years,
been further triggered or exacerbated by other factors, and in
situations where there was a limited conceptualization of
putative causative factors and imperfect knowledge prevented
precise association of causality to expected risk.11 These are
the very issues that confront the investigation of the root
causes of cardiovascular diseases. Thus, when it became
increasingly evident that tobacco abuse was potentially harmful,
a more refined set of principles was used in the 1964 US
Department of Health, Education, and Welfare study on the
association of tobacco and health risks.12 In these criteria,
validation arose from the strength of association between
factor and disease, the presence of a dose-response, lack of
temporal ambiguity between exposure and disease, the fact
that elimination or modification of the putative cause or host
response would eliminate or reduce the disease, and the
biological plausibility of the hypothesis.11,13 In short, it was
suggested that association requires both statistical and scientific
validation.