Describe the design of a case-control study and how it is used to identify the cause of an epidemic.

In a case-control study, each individual affected with a disease (a "case") is paired with an individual that is unaffected by the disease (a "control"). Each case-control pair should be as similar as possible in terms of age, sex, place of residence, and other factors that the investigators consider important. The only significant difference should be whether or not they are affected by the disease. Investigators then look for an additional characteristic or behavior that is common to all the cases, but is not observed in the controls. If such a factor is found, it can suggest to investigators why the cases were at risk for the disease while the controls were not. This information might suggest characteristics of the disease such as its mode of transmission that can help investigators narrow down the search for the disease agent.