Strength of association: the stronger the association between a putative cause and effect, the more likely there is to be a connection between the two.
Timing: The cause should precede the effect. Never the other way around.
Distribution: The spatial distribution of the disease should approximate to the spatial distribution of the causal agent.
Gradient: The response should correlate with the dose; more dose=more effect.
Consistency: The cause-effect relationship needs to be duplicated in different studies in different places.
Specificity: People not exposed to the cause should not develop the effect.
Biological plausibility: the biological activity of the suspected cause should be consistent with its effects.
Experimental models: animal or other experimental models should demonstrate the effect under experimental conditions.
Preventive trials: removal of the suspected cause should lead to removal of the effect.