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Influenza

• Influenza, commonly called "the flu," is a highly contagious viral infection of the respiratory tract.
• Influenza is spread from person to person through airborne droplets from the nose or throat of an infected person.
• Compared with most other viral respiratory infections, such as the common cold, influenza infection often causes a more severe illness.
• Most people who get influenza recover completely in 1 to 2 weeks, but some people develop serious complications, such as pneumonia. The elderly and persons with chronic health problems are most at risk for developing serious complications after influenza infection.
• The best way to prevent influenza is to get a yearly flu shot.

Influenza, commonly called "the flu," is a highly contagious infection of the nose, throat, bronchial tubes, and lungs.

Influenza is caused by viruses that infect the respiratory tract. Two main types of influenza viruses -- influenza type A and influenza type B -- cause the outbreaks and epidemics of respiratory illness that occur almost every winter.

Influenza viruses are unusual because they are always changing. Influenza viruses can undergo two types of changes. "Antigenic drift" is a series of changes over time that cause a gradual evolution of the virus. Antigenic drift results in the emergence of new influenza strains. Different strains circulating during each influenza season give rise to yearly outbreaks and epidemics. "Antigenic shift" is an abrupt change that results in new forms (subtypes) of the virus. Antigenic shift occurs only occasionally. When it does occur, large numbers of people, and sometimes the entire population, are without protective immunity. This can result in a catastrophic worldwide epidemic, called a pandemic, such as those that occurred in 1918, 1957, and 1968.

Influenza occurs worldwide. The major types of influenza virus live and change inside animals, primarily birds, pigs, and horses.

Influenza viruses spread through the air, most often when an infected person sneezes, coughs, or speaks. Influenza is highly contagious and is easily spread from person to person by droplets from the nose or throat of an infected person.

Compared with most other viral infections of the respiratory tract, such as the common cold, influenza infection often causes a more severe illness. Typical symptoms are:

• Abrupt fever
• Muscle aches
• Severe tiredness
• Cough
• Sore throat
• Runny or stuffy nose
• Headache

Influenza also lingers longer than most other common respiratory infections, often lasting a week or more. Influenza should not be confused with the so-called "stomach flu," which is a catch-all term for various digestive system problems caused by other microorganisms.

Symptoms typically appear 1-5 days after infection.

Usually a health-care provider will diagnose influenza based on typical symptoms of fever, chills, headache, cough, and body aches. Laboratory tests to confirm influenza are costly and time consuming and are usually used only for outbreak investigations.

Anyone can get influenza, but the risk of complications is highest is these groups:

• Persons aged 65 years and older
• Residents of nursing homes and other long-term care facilities
• Adults and children with long-lasting disorders of the lungs or heart, including children with asthma
• Adults and children with diabetes, kidney disease, or weakened immune systems
• Women who will be in the second or third trimester of pregnancy during influenza season
• Health-care workers, household members, and others who are in contact with persons at high risk for influenza and influenza-related complications

Most people who get influenza recover completely in 1 to 2 weeks. However, some people develop serious and potentially life-threatening complications such as pneumonia. Influenza-related complications can occur at any age. However, the elderly and persons with chronic health problems are much more likely to develop serious complications after influenza infection than are younger, healthier people.

There is no cure for influenza. Rest and liquids are the main treatments. The antiviral drugs amantadine and rimantadine may prevent or reduce the severity of influenza type A, but they are not effective against type B. Another antiviral drug, zanamivir, was approved by the Food and Drug Administration (FDA) in July 1999 for the treatment of influenza infections caused by both type A and type B viruses.

Influenza occurs to some extent every year. However, because influenza is not caused by a single organism but by a group of related viruses that are constantly changing, each year brings the threat that a pandemic strain might emerge. In 1997, an outbreak of avian influenza ("bird flu") in Hong Kong raised concerns about a possible global pandemic. Although the disease was contained, the outbreak initiated actions to prepare the United States and the world for a possible pandemic that may materialize in the future. The worst pandemic occurred in 1918 and killed more than 20 million people. Subsequent pandemics in 1957 and 1968 also resulted in large numbers of deaths.

Between pandemics, smaller but still substantial influenza epidemics are common. In 1994, an estimated 90 million Americans (about 35% of the population) contracted the flu. More typically, influenza strikes 10% to 20% of the U.S. population yearly. In an average year, influenza is associated with about 20,000 deaths nationwide and many more hospitalizations.

The best way to prevent influenza is to get a yearly flu vaccination. Influenza viruses change over time, and each year the vaccine is updated to include the viruses that are most likely to circulate in the upcoming influenza season. Therefore, people who need to be protected against influenza should get a flu shot every year. Flu shots are 70%-90% effective in preventing influenza in healthy adults. In elderly or chronically ill persons, influenza vaccine may be less effective in preventing illness than in preventing serious complications and death.

Anyone who wants to avoid influenza can get a flu shot. Flu shots are particularly recommended for persons who are at high risk of having a serious complication when they get influenza. These include:

• All persons age 65 years and older
• Persons of any age with certain long-term health problems that put them at increased risk of influenza-related complications, hospitalization, and death
• Care givers of high-risk persons and persons who live with persons at high risk

The best time to get a flu shot is between October and mid-November every year. However, vaccinations can be given as early as September and well into December, even after influenza begins appearing in a community.

This fact sheet is for information only and is not meant to be used for self-diagnosis or as a substitute for consultation with a health-care provider. If you have any questions about the disease described above, consult a health-care provider.

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