Flu, known medically as influenza, is a respiratory infection caused by the influenza virus. The infection typically is spread by air or by direct contact from one person to another. Most cases occur during epidemics, which peak during the winter months nearly every year. Influenza virus is very contagious. A particularly widespread and severe epidemic is called a pandemic.

Compared with other viruses, influenza can strike remarkably large numbers of people in a relatively short time. Each year, about 25 million people seek medical care for the symptoms of flu during flu season. In the United States, the Asian flu of 1957-1958 caused 70,000 deaths, and the Hong Kong flu of 1968-1969 killed 34,000 people. In the worst recorded pandemic of influenza, the 1918-1919 Spanish flu, 20 to 40 million people throughout the world died in less than one year.

The most common types of influenza virus are A and B. Influenza A is the one usually responsible for the annual epidemics. Most people get multiple flu infections during their lives. With many other types of infections – for example, mumps – having the disease once protects against a second infection because the body's immune system "remembers" the returning virus, attacks it immediately and rapidly eliminates it. With influenza, the virus usually has mutated (changed) somewhat since the first infection, and the change is enough to fool your immune system. Instead of attacking the virus rapidly, as it would a virus that it had seen before, the immune system responds slowly. By the time the immune response is in full gear, millions of the body's cells already have been infected with the virus.

Flu can cause a variety of symptoms. They can be mild or severe depending on the type of virus and your age and overall health. Although it is a respiratory virus, flu can affect other body systems, making you feel sick all over. Symptoms can include any or all of the following:

• Chills
• Moderate to high fever (101 degrees to 103 degrees Fahrenheit)
• Sore throat
• Runny nose
• Muscle aches
• Headaches
• Fatigue
• Cough
• Diarrhea
• Dizziness

Dangerous complications also can develop from flu. One of the most feared complications, a bacterial "superinfection," occurs when the influenza virus attacks a lung and weakens its defenses, allowing bacteria that normally live quietly in the nose and throat to descend into the lung and cause bacterial pneumonia. People older than 50, infants, and those with certain chronic diseases or suppressed immune systems are especially vulnerable to complications.

Your doctor will evaluate your symptoms to determine whether you have flu or a cold. If you are sneezing and have a stuffy nose, you may have a cold. Flu is more likely to cause fever, coughing, chills and muscle aches. It also is more seasonal than colds. Flu tends to occur during winter months, whereas colds occur all year long.

Most of the time, doctors assume the diagnosis is flu when you have symptoms of influenza in the winter. If your symptoms or findings on physical examination suggest something other then the flu, your doctor may order blood tests and swab your nose and throat to send secretions to the laboratory for specific influenza testing. Your doctor may order a chest X-ray if he or she suspects that the influenza virus has caused a lung infection (pneumonia) or may lead to a bacterial superinfection.

Influenza symptoms can last for as few as 24 hours or for a week or more. A typical case lasts four or five days. As long as you have symptoms, you are contagious and can spread the infection to others.

Options for heading off an attack of influenza have increased steadily in recent years.

• Vaccination — Vaccination each year is recommended for all people, including school-aged children, who want to reduce the risk of becoming ill with influenza or of transmitting influenza to others. Vaccination is particularly recommended for:
  • All children and adolescents aged 6 months to 18 years of age, especially those receiving long-term aspirin therapy who therefore might be at risk for experiencing Reye's syndrome after influenza infection;
  • All people aged older than 50 years;
  • Women who are pregnant or will be pregnant during the influenza season;
  • Adults and children who have chronic lung (including asthma), heart (except hypertension), kidney, liver, blood or metabolic disorders (including diabetes mellitus);
  • Adults and children whose immune systems are suppressed (because of immunosuppressive medications or by human immunodeficiency virus);
  • Adults and children who have any condition (e.g., cognitive dysfunction, spinal cord injuries, seizure disorders, or other neuromuscular disorders) that can compromise lung function or the handling of respiratory secretions or that can increase the risk for aspiration;
  • Residents of nursing homes and other chronic-care facilities;
  • Health-care personnel;
  • Adults or children who are in close contact with children aged less than 5 years (especially children aged less than 6 months) and adults aged more than 50 years; and
  • Adults or children who are in close contact with people who have medical conditions that put them at higher risk for severe complications from influenza.

  The standard flu vaccine is 70% to 90% effective in helping healthy people younger than 65 to avoid the disease or lessen its severity. For maximum effectiveness, doctors advise people to get vaccinated in October or November, the start of flu season.

  Healthy people between the ages of 2 and 49 have an alternative to the flu shot. FluMist is an intranasal vaccine spray that appears to offer similar protection. It uses a deactivated live virus rather than the killed virus in the shot. FluMist is not any more effective than the standard flu shot. Because FluMist is so new, people at the highest risk for flu (people older than 49 and those with chronic health conditions) should receive the injected vaccine.

• Good hygiene — The virus usually is passed through the air, by coughing. It also is passed by direct contact, such as shaking hands or kissing. For this reason, practicing good hygiene – covering your mouth when you cough and washing your hands frequently – can help you to avoid getting the flu or spreading it to others.



• Antiviral drugs — Zanamivir (Relenza) and oseltamivir (Tamiflu) can reduce your chance of getting the flu by 70% to 90% if they are taken just before an expected outbreak. Zanamivir is a spray that is inhaled through the mouth. The others are pills.

  The older drugs previously used to help prevent influenza, amantadine and rimantadine, have lost their effectiveness. But zanamivir and oseltamivir remain active against most strains of influenza A and B.

  Zanamivir is given by inhalation from a nebulizer. It is approved for prevention in people ages 5 and older and for treatment in people ages 7 and older. Side effects may include nausea, and vomiting and wheezing, especially in people with asthma or chronic lung disease.

  Oseltamivir is available in tablet form. It is approved for prevention and treatment in patients older than one year. Side effects may include nausea and vomiting.

To ease symptoms, your doctor will recommend that you rest and drink plenty of fluids (at least eight cups daily). For fever and body aches, you can take over-the-counter pain relievers. If zanamivir (Relenza) or oseltamivir (Tamiflu) are taken within 48 hours of the start of symptoms, they may speed recovery by about one day. Because flu is a viral infection, antibiotics are not effective. Children who are suspected of having flu, and who have high fevers should never be given aspirin to treat the fever. This can cause the disease called Reye's syndrome. Instead, acetaminophen (Tylenol) should be used.

If you have a chronic disease and suddenly get flu symptoms, you should call your doctor's office, because you may benefit from starting an antiviral medication within 48 hours. You also should notify your doctor if you have flulike symptoms along with chest pain, ear pain, shortness of breath, fever that does not go away or a cough that produces blood or thick, foul-smelling mucus.

Although most people recover fully from the flu, some develop serious complications, including life-threatening conditions such as pneumonia. About 20,000 people in the United States die of flu complications each year, and thousands more need to be hospitalized prior to recovery.

Centers for Disease Control and Prevention (CDC)
1600 Clifton Road
Atlanta, GA 30333
Phone: 404-639-3534
Toll-Free: 1-800-311-3435
http://www.cdc.gov/