Whereas medicine is concerned with the study, treatment and - to a lesser extent - prevention of disease in people, epidemiology is concerned with the study of disease in populations.
A doctor might notice that an individual suffered from influenza every year and might recommend that they be immunized or that they change their lifestyle to reduce their exposure to the influenza virus.
An epidemiologist would look at the rate on influenza attacks on people in various cohorts (ages, occupations, socio-economic class), the impact of those attacks on workforce productivity, demand for hospital services and on mortality to recommend whether immunization should be encouraged generally, targeted, subsidized or made mandatory for certain people.
Epidemiologists give greater weight than do doctors to the social context of disease. In the case of immunization, are there certain groups who have an objection to immunization? If the susceptible people have a low socio-economic status, will it be more difficult to reach them and to encourage them to participate in preventive behaviours? Is the cost of prevention significant enough to impact on its take-up - and therefore on its spread.
Epidemiologists have a huge role to play in understanding diseases which can be contained by behavioural change: alcoholism, AIDS, hepatitis C. In these there is a powerful social pressure to behave in ways which give short-term increases in individual enjoyment or feelings of well-being while, at the same time, increasing exposure to the disease.
* The third syllable is pronounced to rhyme with 'dem' in democracy.