A person may be genetically predisposed to certain diseases, but other factors such as smoking or high cholesterol levels, may be required to bring on the onset. Correlations don't prove a cause and effect relationship. However, that does not prove that smoking causes lung cancer. You would have to show that the correlation between smoking and lung cancer is independent of dwelling place, so urban vs. rural is eliminated as a variable. And so on.
Errors are unavoidable in any experiment.
No measurement is perfect. Systematic errors can not be eliminated by averaging, but if they are detected, they can be corrected for. Since every measurement includes some error, it is essential to provide an estimate of the likely range of errors. In controversial areas, interpretation of the experimental evidence may be too difficult for a consensus to form, at least initially. For that matter, it's important to recall that because science is inductive, all theories are just models, subject to revision or replacement.