-- Whereas a physician tries to determine presence of disease and causes in individuals, epidemiologists focus on populations

-- Unlike microorganisms (like a bacteria) which can be linked to a given disease (disease is defined as exposure to that microorganism) - few chemical/physical factors have a unique effect on health - for example exposure to asbestos - causes lung cancer, but other things may also cause lung cancer

-- Also, outcomes may be due to a combination of factors - e.g., genetics + environmental exposure = disease, so env. exposure is a component cause

-- Different individuals within population with the disease may have gotten it through different causal pathways - one person through env. exposure another through personal factor, etc.