Influenza is a common viral infection. It is caused by three viruses - Influenza A, B and C. Type A is usually responsible for the large outbreaks and is a constantly changing virus. New strains of Type A virus develop regularly and cause new epidemics every few years. Type B causes smaller outbreaks, and Type C usually causes mild illness. In the United States, infection with influenza A and B leads to 20,000 deaths and more than 100,000 hospitalizations each year. Influenza is transmitted from person to person via contagious droplets that are spread when an infected person sneezes or coughs.

Certain individuals are at higher risk from complications of influenza and therefore should be vaccinated:

• People aged 50 or older
• People with conditions such as the following:
  • diabetes
  • conditions affecting the heart
  • conditions affecting the lungs (such as asthma)
  • conditions affecting the kidneys
• Health care workers
• Anyone with a weakened immune system (such as transplant recipients or people with HIV)

Supplies of vaccine are limited each year, but after high-risk people have been vaccinated, anyone desiring protection can request vaccination.

• fever - up to 104 degrees C
• headache
• muscle aches and stiffness
• nasal discharge
• cough
• shortness of breath
• chills
• fatigue
• malaise
• sweating
• loss of appetite
• stuffy, congested nose
• sore throat

The evaluation of an individual with symptoms of influenza should include a thorough physical exam and in cases where pneumonia is suspected, a chest X-ray.

Additional blood work may be warranted during the evaluation which may include a complete blood count, blood cultures and sputum cultures.

The most common methods for diagnosing influenza include antigen detection tests, which are done on nose and throat (nasopharyngeal) secretions by swabbing these areas with a dacron swab and then sending a sample to the laboratory for testing.

The results of these tests can be available rapidly, and can help decide if specific treatment is appropriate, but the diagnosis can often be made by identifying symptoms without further testing.

For mild illness in people who are not at high-risk (not immunosuppressed, elderly, no chronic heart, lung or kidney conditions) - the treatment of influenza is frequently just supportive and includes bed rest, analgesics (pain killers) for muscle aches and pains, and increased intake of fluids.

If influenza is diagnosed within 48 hours of the onset of symptoms, in particular among high-risk groups, several antiviral medications are available which may shorten the duration of symptoms by approximately 1 day. These medications include amantadine or rimantadine (active against influenza A only); oseltamivir and zanamivir (active against influenza A and B). As each of these medicines has different side effects and affects different viruses, your physician will determine which one is best for you.

Treatment is usually not necessary for children, but if the illness is diagnosed early and the patient is at risk of progression to more severe disease, it can be started. Oseltamivir (Tamiflu) is the best choice for children. It is available in a liquid formulation and may be easier to give to the child than zanamivir (Relenza) which is not licensed for children less than 12 years old and needs to be given by via inhaler. Treatment will only help if started early and only if the illness is actually influenza, it will not help treat a "regular cold."

In most individuals who are otherwise healthy, influenza fully resolves within 7 to 10 days. Among individuals in high-risk groups (elderly, immunosuppressed, chronic heart, lung or kidney conditions) influenza may be quite severe and can lead to complications listed below.

• secondary bacterial infection
• pneumonia
• encephalitis (infection of the brain)

Call your health-care provider if you develop symptoms of influenza. While many infections have similar symptoms, if you belong to a high-risk group, you should be evaluated for influenza and your physician will assess whether you should be on antiviral medications.

Flu shots are recommended annually for people who are 50 years of age or older, anyone with chronic heart, lung or kidney conditions, and those living in institutions. The vaccine has a 60% to 70% success rate in preventing infection among individuals with normal immune systems; efficacy is lower in individuals with weakened immune systems. The influenza vaccine should not be given to people who are allergic to eggs.

Approximately 8 million children and adolescents between 6 months and 17 years of age have one or more medical conditions that put them at increased risk of influenza-related complications and should be given the first vaccine available:

• Children with chronic disorders of the heart or lungs (such as asthma and cystic fibrosis)
• Children who have required regular medical follow-up or hospitalization during the preceding year because of conditions such as the following:
  • chronic metabolic diseases (including diabetes)
  • kidney dysfunction
  • sickle cell anemia
  • immunosuppression
• Children and teenagers receiving long-term aspirin therapy and therefore might be at risk for developing Reye's syndrome after influenza infection
• Adolescent girls who will be in the second or third trimester of pregnancy during the influenza season

For unvaccinated individuals who have been exposed to people with known influenza, especially if the exposed individual has risk factors, potential use of antiviral medication for more than 2 weeks and vaccination may help prevent illness.