Seasonal influenza, commonly called "the flu," is caused by influenza viruses, which infect the respiratory tract (i.e., the nose, throat, lungs). Unlike many other viral respiratory infections, such as the common cold, the flu can cause severe illness and life-threatening complications in many people. In the United States, on average 5% to 20% of the population gets the flu and more than 200,000 people are hospitalized from seasonal flu-related complications. Flu seasons are unpredictable and can be severe. Over a period of 30 years, between 1976 and 2006, estimates of flu-associated deaths range from a low of about 3,000 to a high of about 49,000 people. Some people, such as older people, young children, pregnant women, and people with certain health conditions, are at high risk for serious flu complications. The best way to prevent seasonal flu is by getting a seasonal flu vaccination each year.
In 2009-2010, a new and very different flu virus (called 2009 H1N1) spread worldwide causing the first flu pandemic in more than 40 years. CDC expects the 2009 H1N1 virus to spread again this season, along with other seasonal flu viruses. This season’s flu vaccine will protect against three different flu viruses: an H3N2 virus, an influenza B virus and the H1N1 virus that caused so much illness last season. You should get vaccinated this year even if you got a 2009 H1N1 or a seasonal vaccine last year because the vaccine viruses have been updated. Immunity sets in about two weeks after vaccination and the flu vaccine provides protection that lasts through the flu season.
The flu can cause mild to severe illness, and at times can lead to death. The flu is different from a cold. The flu usually comes on suddenly. For information about flu symptoms, see Flu Symptoms & Severity.
In the United States, the peak of flu season has occurred 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.
CDC collects data year-round and reports on influenza (flu) activity in the United States each week from October through May. The U.S. influenza surveillance system consists of seven separate components.
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.
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.
The main way that influenza viruses are thought to spread is from person to person in respiratory droplets of coughs and sneezes. For more information about flu transmission, visit How Flu Spreads.
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 2009 H1N1 virus that predominated during the 2009-10 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 2010-11 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 2009-10 flu season, 2009 H1N1 viruses predominated; however, infection with this virus (and subsequent antibodies protecting against re-infection with the same virus) would not provide protection against influenza B or influenza A (H3N2) viruses.
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. For more information, see Flu Symptoms & Severity.
It is very difficult to distinguish the flu from other viral or bacterial causes of respiratory illnesses on the basis of symptoms alone. There are tests available to diagnose flu. For more information, see Diagnosing Flu.
In addition to flu viruses, several other respiratory viruses also can circulate during the flu season and can cause symptoms and illness similar to those seen with flu infection. These non-flu viruses include rhinovirus (one cause of the "common cold") and respiratory syncytial virus (RSV), which is the most common cause of severe respiratory illness in young children as well as a leading cause of death from respiratory illness in those aged 65 years and older.
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.
Information about how long a person is contagious is available at How Flu Spreads.
Flu seasons are unpredictable and can be severe. It is estimated that, on average, approximately 5% to 20% of U.S. residents get the flu, and more than 200,000 people are hospitalized for flu-related complications each year. Over a period of 30 years, between 1976 and 2006, estimates of flu-associated deaths range from a low of about 3,000 to a high of about 49,000 people.
Yes. There are influenza antiviral drugs that can be used to treat flu illness.
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.
For more information, please visit CDC Website.