Encyclopedia / I / Influenza


What is Influenza?

Influenza, also known as the flu, is a common, acute, highly contagious viral infection of the respiratory tract that tends to peak during the winter in the United States. Often occurring as an epidemic, the flu results from three kinds of Orthomyxoviruses: influenzae types A, B, and C. Type A viruses tend to be the most virulent, and constantly mutate into new strains, making permanent immunization against them difficult. Type A influenza is most often responsible for large epidemics. The type B virus, generally a less severe variety, mutates occasionally, causing smaller, more localized outbreaks.

Type C, which causes only mild illness resembling a common cold, is a very stable virus. During an initial bout with a type C infection, the body produces antibodies that confer immunity against future infections. Symptoms of all types of influenza usually appear following an incubation period of one to four days after initial exposure, and run their course over a period of seven to 10 days.

Who Gets Influenza?

Anyone can come down with influenza. Every year, between 5 to 20 percent of the United States population gets the flu, and more than 200,000 people are hospitalized from seasonal flu-related complications. The elderly, children, pregnant women, patients living in long-term facilities, and the chronically ill are at increased risk of potentially serious flu-related complications, such as bacterial bronchitis and pneumonia.


  • Sudden onset of chills and fever, usually between 101°F and 104°F. Fever may last three to five days.

  • Muscle aches; headache.

  • Joint pain, achiness

  • Dry cough; sore throat and hoarseness.

  • Loss of appetite.

  • Nasal congestion or runny nasal discharge.

  • Chest pain.

  • Burning sensation in the eyes.

Causes/Risk Factors

Everyone is at risk of developing influenza, particularly if there is an outbreak. The virus is spread by infected people coughing or sneezing. The droplets travel through the air and can be inhaled by others. You can also catch flu from direct contact—touching hands or kissing. Influenza viruses can even live for hours in dried mucus that may have been left on objects touched by a flu-infected person’s unwashed hands. Outbreaks typically occur in winter, when people tend to remain inside and are in close proximity to one another, although influenza can occur year-round.

What If You Do Nothing?

Flu is a self-limiting ailment, generally not dangerous, and will normally clear by itself within 10 days or less—though weakness and fatigue can persist for several weeks or longer. However, influenza can be life-threatening for high-risk individuals, including children, the elderly, and people with chronic medical conditions.


Tests are not necessary to diagnose the flu. However, a viral culture swab can be done to confirm a flu diagnosis or to determine a particular strain of flu.


For healthy adults and children, influenza can resolve on its own with proper home remedies, plenty of rest and consuming plenty of liquids to reduce the risk of dehydration. There are also approved medications to treat influenza.

  • Get plenty of bed rest, preferably in a warm, well-ventilated room. Using a cool-mist humidifier may help to thin mucus.
  • Over-the-counter pain relievers ease aches and reduce fever. Give acetaminophen, not aspirin, to children. Aspirin should not be used in children younger than 18 years of age due to an increased risk forReye's syndrome. Reye's syndrome is a serious complication that can develop following a viral infection.
  • Drink plenty of fluids to keep mucus secretions thin and easy to expel. Sucking on cough suppressants may also help.
  • Drink warm liquids, such as hot tea with honey, or gargle with salt water to alleviate a sore throat. Avoid drinking alcohol or caffeine since this can contribute to dehydration.
  • Antiviral drugs, if taken within two days of the onset of the flu, may reduce the length and severity of the illness. The U.S. Food and Drug Administration has approved four antiviral drugs (amantadine [Symmetrel], rimantadine [Flumadine], zanamavir [Relenza], and oseltamivir [Tamiflu]) for treatment of influenza. Amantadine or rimantadine may help reduce the severity of symptoms from type A infections, if administered within 24 hours of onset. Zanamivir or oseltamivir can be used to treat type A and type B infections.


The best way to protect against influenza is to get vaccinated. Influenza vaccines are often free and administered at doctors' offices, clinics and community settings every fall, right before the winter flu season begins. However, the flu vaccine is only about 60 percent effective, meaning that getting the vaccine greatly reduces the chances of developing flu, but doesn't provide 100 percent protection.

The Centers for Disease Control and Prevention (CDC) now recommends the annual influenza vaccine to everyone over six months of age. An annual flu vaccine is especially recommended for people with chronic medical conditions, including those with chronic heart or kidney disease; chronic lung disease (including asthma, emphysema, chronic bronchitis, tuberculosis, or cystic fibrosis; diabetes mellitus or other chronic metabolic disorders; severe anemia; and depressed immunity due to diseases, such as AIDS, or medical treatments, such as chemotherapy for cancer. Women who will be in the second or third trimester of pregnancy during influenza season, health-care workers, and people over age 50 or who live in a nursing facility are also at heightened risk.

The vaccine should be administered before the start of flu season, between October 1 and the middle of November. Since it takes one to two months after being vaccinated to build up sufficient antibodies, mid-October to mid-November is the best time to get flu shots—well before the start of the annual late-December flu season. Flu vaccine contains the flu viruses expected to cause illness that year, so new batches must be created at the start of each flu season.

Some, but not all flu vaccines, contain trace amounts of egg. The CDC cautions that anyone with severe allergic reaction to eggs may be advised not to get vaccinated. People who have experienced a mild reaction to egg, such as hives, may receive a flu shot with additional precautions. Persons over the age of 65 can be offered a high-dose form of vaccine.

The CDC recommends a nasal spray vaccine for healthy children ages 2-8. (Children with illnesses that would make them more likely to suffer from one of the complications of influenza would be given the standard flu shot.)

Other ways to reduce the risk for flu involve avoiding close contact with infected people and maintaining a healthy lifestyle. Regular handwashing with warm water and soap or alcohol-based sanitizer may help reduce viral transmission. Avoid touching the eyes, nose or mouth. Germs can spread this way. Also, cover your mouth and nose with a tissue when sneezing or coughing. Discard the tissue in the bin after use. Show respect to your co-workers by not going to work when you have the flu or other contagious diseases.

When To Call A Doctor

See a doctor if flu symptoms do not improve within a week. Individuals with any of the conditions that carry a high risk of complications should consult a doctor immediately if flu exacerbates an underlying illness or if flu symptoms are severe.

Robert Hurd, M.D., American Board of Internal Medicine and Professor of Endocrinology and Health Care Ethics, Xavier University, Cincinnati, OH. Review provided by VeriMed Healthcare Network.