The epidemiologist here at the Hartford Health Department is called on when an epidemic takes place in Hartford. Thankfully, though, things like that don't happen every day. Like we said earlier, epidemiologists look at disease or illness on a population scale. But you can't examine or give physicals to everyone in Hartford. For epidemiologist to do their work then, they have to look at data.
Data is nothing more than information.
Of course, it helps if that information is organized in one way or another. That way, it makes it easier to look at. Sources of data can be practically anything you can think of. These data then let us try and look for differences in groups of people contianed within the data. Epidemiology is the science that allowed the Surgeon General to determine that smoking was bad for your health in the 1950's.