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Table 1 The five categories of ILI surveillance used by the CDC

From: How many suffice? A computational framework for sizing sentinel surveillance networks

Category Description
Viral surveillance Approximately 85 U.S. World Health Organization (WHO) Collaborating Laboratories and 60 National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories located throughout the United States participate in virologic surveillance for influenza.
Outpatient illness surveillance Information on patient visits to health care providers for influenza-like illness is collected through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet). ILINet consists of more than 2,700 outpatient healthcare providers in all 50 states, the District of Columbia and the U.S. Virgin Islands reporting more than 30 million patient visits each year.
Mortality surveillance 122 cities across the United States report the total number of death certificates and the number for which pneumonia or influenza was listed as the underlying or contributing cause of death. Additionally, all influenza-associated deaths in children (age < 18) are reported.
Hospitalization surveillance Laboratory confirmed influenza-associated hospitalizations in children and adults are monitored through the Influenza Hospitalization Surveillance Network (FluSurv-NET).
Summary of the geographic spread of influenza State health departments report the estimated level of geographic spread of influenza activity in their states each week spread of influenza through the State and Territorial Epidemiologists Reports.