Influenza-like Illness (ILI)

An influenza ILINet provider conducts surveillance for influenza-like illness (ILI) in collaboration with the state health department and the Centers for Disease Control and Prevention. Data reported by ILINet providers, in combination with other influenza surveillance data, provide a national picture of influenza virus and ILI activity in the United States. The Outpatient Influenza-like Illness Surveillance Network (ILINet) consists of more than 2,900 healthcare providers in all 50 states, Puerto Rico, the District of Columbia, and the U.S. Virgin Islands. 

ILINet providers report each week the total number of patient seen for any reason and the number of patient visits for influenza-like illness (ILI) by age group (0-4 years, 5-24 years, 25-49 years, 50-64 years, >65 years).

Influenza-like Illness (ILI) Case Definition

• Fever (= 100° F [37.8° C], oral or equivalent)
• Cough and/or sore throat 

These data are transmitted once a week via the internet or fax to a central data repository at CDC. Most providers report that it takes them fewer than 30 minutes a week to compile and report their data.

Providers of any specialty in any type of practice are eligible to be ILINet providers.

• Emergency Medicine
• Family Practice
• Infectious Disease
• Internal Medicine
• OB/GYN
• Pediatrics
• Student Health
• Urgent Care 

Practice settings that are not eligible are elementary, middle, or high school health centers, and any type of institutional setting such as nursing homes or prisons.

Influenza viruses are constantly evolving and cause substantial morbidity and mortality; flu associated deaths range from 3,000 to a high of about 49,000 each year. ILINetdata is critical for monitoring the course of novel influenza A (H1N1) virus activity on the local, state, and national level. Also, ILINet data, in combination with other influenza surveillance data, can be used to guide prevention and control activities, vaccine strain selection, and patient care. The most important consideration is that the data provided are critical for protecting the public's health.