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Questions & Answers
Seasonal Influenza
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 has occured anywhere from late November 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.
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.
How does the flu spread?
The main way that influenza viruses are spread is from person to person in respiratory droplets of coughs and sneezes. (This is called "droplet spread.") This can happen when droplets from a cough or sneeze of an infected person are propelled (generally up to 3 feet) through the air and deposited on the mouth or nose of people nearby. Though much less frequent, the viruses also can be spread when a person touches respiratory droplets on another person or an object and then touches their own mouth or nose (or someone else’s mouth or nose) before washing their hands.
If I got the flu last year, will I have immunity against the flu this year?
In general, a person who is infected with an influenza virus one year will have some immunity to closely related viruses that may persist for one or more years. For example, if someone was infected with the A/New Caledonia/20/99-like strain of H1N1 that predominated during the 2006-07 season, they are likely to have some immunity that will protect them if they are exposed to that strain or a closely related strain again during the 2007-08 season. The degree of protection depends on the health of the person involved. Young and healthy people with normal immune systems will likely have good immunity against the same or closely related strains of virus from one year to the next. However, people with weakened immune systems are less likely to have immunity that carries over in other years. It's important to remember that influenza viruses are constantly changing so antibody made against one strain will become less effective against new strains as influenza strains evolve over time. In addition, there are different types of influenza viruses circulating and different variants within virus types, and the same type of flu virus does not necessarily circulate each year. For instance, during the 2006-07 flu season, influenza A (H1N1) viruses predominated; however, infection with an influenza A (H1N1) virus (and subsequent antibodies protecting against re-infection with the same virus) would not provide protection against influenza B or influenza A (H3N2) viruses.
Does the flu have complications?
Yes. Some of the complications caused by flu include bacterial pneumonia, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes. Children may get sinus problems and ear infections as complications from the flu. Those aged 65 years and older and persons of any age with chronic medical conditions (such as asthma, diabetes, or heart disease) are at highest risk for serious complications of flu.
How do I find out if I have the flu?
It is very difficult to distinguish the flu from other viral or bacterial causes of respiratory illnesses on the basis of symptoms alone. A test can confirm that an illness is influenza if the patient is tested within the first two to three days after symptoms begin. In addition, a doctor's examination may be needed to determine whether a person has another infection that is a complication of influenza.
How soon will I get sick if I am exposed to the flu?
The time from when a person is exposed to flu virus to when symptoms begin is about one to four days, with an average of about two days.
How long is a person with flu virus contagious?
The period when an infected person is contagious depends on the age and health of the person. Studies show that most healthy adults may be able to infect others from 1 day prior to becoming sick and for 5 days after they first develop symptoms. Some young children and people with weakened immune systems may be contagious for longer than a week.
How many people get sick or die from the flu every year?
Each flu season is unique, but it is estimated that, on average, approximately 5% to 20% of U.S. residents get the flu, and more than 200,000 persons are hospitalized for flu-related complications each year. About 36,000 Americans die on average per year from the complications of flu.
Is the “stomach flu” really the flu?
No. Many people use the term "stomach flu" to describe illnesses with nausea, vomiting or diarrhea. These symptoms can be caused by many different viruses, bacteria or even parasites. While vomiting, diarrhea, and being nauseous or "sick to your stomach" can sometimes be related to the flu – more commonly in children than adults – these problems are rarely the main symptoms of influenza. The flu is a respiratory disease and not a stomach or intestinal disease.
- Page last modified July 16, 2008
- Content Source: Coordinating Center for Infectious Diseases (CCID)
- National Center for Immunization and Respiratory Diseases (NCIRD)
