Be systematic and comprehensive in case finding. Case finding is not only to identify cases for a case-control or other analytic study, but to determine the extent of the outbreak (who is affected? are there milder cases than those that might have been the first to be reported? what is the geographic extent?)

Sources might include:

- asking persons with the disease (“case-patients”) if they know anyone else who has a similar illness

- depending on the disease, contacting physicians, clinics, emergency departments, hospitals, university infirmaries, school nurses, and/or whichever other sites and health care providers might see persons with the illness