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What is influenza (flu)?
Influenza, commonly called "the flu," is caused by the influenza virus, which infects the respiratory tract (nose, throat, lungs). Unlike many other viral respiratory infections, such as the common cold, the flu causes severe illness and life-threatening complications in many people.
What are the symptoms of the flu?
Influenza is a respiratory illness. Symptoms of flu include fever, headache, extreme tiredness, dry cough, sore throat, runny or stuffy nose, and muscle aches. Children can have additional gastrointestinal symptoms, such as nausea, vomiting, and diarrhea, but these symptoms are uncommon in adults. Although the term "stomach flu" is sometimes used to describe vomiting, nausea, or diarrhea, these illnesses are caused by certain other viruses, bacteria, or possibly parasites, and are rarely related to influenza. Please also see "Is it a Cold or the Flu".
When is the flu season in the United States?
In the United States, the peak of flu season can occur anywhere from late December through March. The overall health impact (e.g., infections, hospitalizations, and deaths) of a flu season varies from year to year. CDC monitors circulating flu viruses and their related disease activity and provides influenza reports each week from October through May. See Weekly U.S. Influenza Summary Update.
How does CDC monitor the progress of the flu season?
CDC collects data year-round and reports on influenza (flu) activity in the United States each week from October through May. These reports are available at www.cdc.gov/flu/weekly/fluactivity.htm. The U.S. influenza surveillance system consists of seven separate components.
- Laboratory-based viral surveillance, which tracks the number and percentage of influenza-positive tests from laboratories across the country;
- Sentinel physician surveillance for influenza-like illness ( ILI ), which tracks the percentage of doctor visits for flu-like symptoms;
- Mortality surveillance as reported through the 122 Cities Mortality Reporting System, which tracks the percentage of deaths reported to be caused by pneumonia and influenza in 122 cities in the United States;
- State and territorial epidemiologist reports of influenza activity, which indicates the number of states affected by flu and the degree to which they are affected;
- Influenza-associated pediatric mortality as reported through the Nationally Notifiable Disease Surveillance System, which tracks the number of deaths in children with laboratory confirmed influenza infection; and
- Influenza-associated pediatric hospitalizations as reported through the Emerging Infections Programs in 9 sites which tracks the number of children reported hospitalized for flu-related complications; and
- Influenza-associated pediatric hospitalization as reported through the New Vaccine Surveillance Network in 3 sites, which also tracks the number of children reported hospitalized for flu-related complications.
These surveillance components allow CDC to determine when and where influenza activity is occurring, determine what types of influenza viruses are circulating, detect changes in the influenza viruses collected and analyzed, track patterns of influenza-related illness, and measure the impact of influenza in the United States . All influenza activity reporting by states, laboratories, and health-care providers is voluntary. For more information about CDC's influenza surveillance activities, see the Overview of Influenza Surveillance in the United States.
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Why is there a week-long lag between the data and when it's reported?
The influenza surveillance system is one of the largest and most timely surveillance systems at CDC. The system consists of 7 complementary surveillance components. These components include reports from more than 120 laboratories, 2,000 sentinel health care providers, vital statistics offices in 122 cities, research and health care personnel at the Emerging Infections Program (EIP) and New Vaccine Surveillance Network (NVSN) sites, and influenza surveillance coordinators and state epidemiologists from all 50 state health departments and the New York City and District of Columbia health departments. Influenza surveillance data collection is based on a reporting week that starts on Sunday and ends on Saturday of each week. Each surveillance participant is requested to summarize weekly data and submit it to CDC by Tuesday afternoon of the following week. The data are then downloaded, compiled, and analyzed at CDC each Wednesday. The compiled data are interpreted and checked for anomalies which are resolved before the report is written and submitted for clearance at CDC. On Friday the report is approved, distributed, and posted to the Internet.
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