A process for establishing causation between an agent and a disease, Koch’s postulates were first developed for studies of infectious disease agents.
Early on, the assumption was that a single exposure lead to a certain disease or trait. That was replaced by the epidemiologic concept of multi-causality; that things aren’t always that simple. Scientists would recognize that many diseases have multiple causes.
Diseases are not caused by environmental factors alone, but also by constitutive factors that have to do with genetics and other forms of conditioning and inheritance.