factors had been identified, disease control efforts could be modified accordingly.

The importance of flexibility in surveillance activities was underscored early in the eradication campaign. The initial strategy had been to conduct mass vaccinations in every endemic country and at the same time improve surveillance capabilities. It was felt that once 80 percent of a country's population was immunized, any remaining foci of infection could be rapidly identified, contained, and eliminated.

Once the campaign was under way, however, it became clear that achieving the 80 percent immunization goal might not be necessary. A more targeted approach, called surveillance-containment, was tried. Infected individuals were located and isolated, and known or suspected contacts were vaccinated, thus preventing the disease from spreading to others. The new strategy worked because smallpox infection is never silent, because it spreads slowly compared with many other infectious diseases, and because vaccination could produce immunity within the incubation period for the disease.

Current U.S. surveillance efforts include both domestic and international components. Although the domestic program, in which a number of federal government agencies participate independently, is fairly comprehensive, U.S. international surveillance activities at this time are fragmented and inadequate to detect emerging infectious disease threats on a timely basis.

Surveillance of infectious diseases in the United States is a passive process. It relies on physicians, hospitals, and other health care providers to report cases to state and local organizations that are responsible for disease surveillance. The Centers for Disease Control (CDC) works in cooperation with the states in monitoring the domestic incidence of specific infectious diseases (such as measles, mumps, rubella, pertussis, diphtheria, and hepatitis B). Each state has its own regulations regarding the reporting of specific diseases. These "notifiable" diseases may duplicate or expand on the list of 49 diseases that are reportable to the CDC (see Table 3-1).

The bulk of the federal reporting requirements are implemented through the National Notifiable Diseases Surveillance System (NNDSS), established in 1961. The list of nationally notifiable diseases is maintained and revised as needed by the Council of State and Territorial Epidemiologists in collaboration