As noted above, surveillance provides information for action. One of the first actions that result from a surveillance case report or report of a cluster is an investigation by the public health department. The investigation may be as limited as a phone call to the healthcare provider to confirm or clarify the circumstances of the reported case, or it may involve a field investigation requiring the coordinated efforts of dozens of people to characterize the extent of an epidemic and to identify its cause.
The objectives of such investigations also vary. Investigations often lead to the identification of additional unreported or unrecognized ill persons who might otherwise continue to spread the infection to others. For example, one of the hallmarks of investigations of persons with sexually transmitted diseases is the identification of sexual partners or contacts of patients. When interviewed, many of these contacts are found to be infected without knowing it and are given treatment they
did not realize they needed. Identification and treatment of these contacts prevent further spread.