The WHO is a focal point for surveillance data on global infectious diseases. Under the International Health Regulations, all countries (except Australia, Papua New Guinea, North Korea, and South Africa) must report to the WHO within 24 hours all cases of cholera, plague, and yellow fever (or any isolation of yellow fever virus from monkeys or mosquitoes). This information is published in the WHO's Weekly Epidemiological Record. Despite the requirement to do so, however, some countries are reluctant to release surveillance data. As a result, some outbreaks of these diseases are never discovered or are discovered only retrospectively after they have subsided.

The WHO also operates a number of networks, composed of selected laboratories worldwide (collaborating centers), that report and investigate outbreaks of specific diseases, such as influenza and HIV disease. The influenza surveillance network is designed to monitor newly emerging strains and subtypes of influenza virus. As noted earlier, the information it collects is used to determine the antigenic makeup of each year's influenza vaccines.

Among other activities, the HIV disease network is encouraging participating countries to do seroprevalence studies at sentinel sites (rather than just reporting numbers of cases) and to develop trend data on infection; it is also collecting geographically representative strains of HIV. More informally, the WHO gathers data about disease outbreaks through its contacts with tourist agencies and international companies, whose clients and employees often inadvertently act as sentinels for new or emerging diseases when they become infected while traveling in other countries.

As is true for many similar efforts, WHO disease surveillance activities are hindered by incomplete reporting and a frequent failure to obtain laboratory confirmation of reported cases of disease. Most cases of yellow fever, for example, are diagnosed on the basis of clinical symptoms alone and often occur in areas in which hepatitis or other tropical diseases with similar symptoms are prevalent. Although the WHO makes every attempt to obtain clinical specimens to allow a definitive diagnosis of reported cases, this is not always possible.

The WHO often is involved in early investigative efforts of newly emerging or reemerging infectious diseases, such as Ebola and Lassa fevers, yellow fever, and dengue fever. For example, when Ebola fever outbreaks occurred in Zaire and Sudan, the WHO provided rapid-response teams, composed of consultants from a number of countries, to help the governments of these nations determine the origin of the outbreaks and develop control strategies.

At one time, the WHO supported a series of serum banks, established in 1960 by John Paul, a physician-epidemiologist at Yale University who is

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