The Centers for Disease Control and Prevention have defined surveillance as follows:

Disease surveillance is the basic process by which epidemiologists answer questions about who, where, and when.

  Who is getting the disease? Are there differences in the rates of disease by age? sex? race?

  Where is the disease happening? Are there geographic areas with particularly high rates? extremely low rates?

  Is the occurrence of the disease changing over time? Is the disease becoming more frequent? less frequent?

Disease surveillance information is useful for:

• Estimating the magnitude of a problem
• Determining the geographic distribution of illness
• Portraying the natural history of a disease
• Detecting epidemics or defining a problem
• Generating hypotheses, stimulating research
• Evaluating control measures
• Monitoring changes in infectious agents
• Detecting changes in health practices
• Facilitating planning

A disease surveillance system should be simple, flexible, and acceptable to the population. For example, to detect hunting-related shooting injuries, the requirements for a hunter to report an episode should not be onerous or many shooting injuries will go unrecorded. The surveillance system should also be representative of the population and provide a timely alarm. Like a smoke detector without a power source, a surveillance system that is not able to recognize a disease outbreak quickly and accurately is not very useful.