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Alternate Names

  • influenza


Influenza, more commonly known as the flu, is a viral illness that is acquired through and primarily infects the respiratory tract but can cause symptoms throughout the body when the person is ill

What is going on in the body?

There are 3 types of influenza viruses: influenza A, B, and C. Type C causes only mild disease in humans, does not occur in epidemics, and is not represented in the annual vaccine. Both types A and B mutate (change their genes slightly) and vary from year to year - type A more so than type B.. This is why it is possible for the same person to have repeated episodes of the flu. Flu is usually transmitted from person to person through drops of respiratory discharge.


What are the causes and risks of the infection?

Because individual cases of influenza are not reportable, it is not known with certainty what fraction of the population is infected during a given influenza season. Any estimate based on office visits or laboratory tests, however, far underestimates the impact.
The severity of a flu season is more accurately measured by hospitalizations and deaths, though these represent a tiny fraction of the total number infected.
Influenza is highly contagious. During flu season, people living in enclosed settings such as dormitories or nursing homes are likely to experience outbreaks. . Those who come in contact with many other people in schools, day care centers, or large group organizations are also more likely to become infected.
In the northern hemisphere, the flu season usually begins in late October or early November and goes through the early spring. In most years, the number of cases peaks in January and February.


What can be done to prevent the infection?

An injection of flu vaccine every year can provide protection for people who want to avoid the flu.
The flu vaccine contains three strains of killed flu virus - two type A strains and one for type B. It is produced roughly 9 to 12 months ahead of the flu season. The formula used is based on what experts believe will be the most common strains of flu virus in the coming season.
The vaccine has a 60% to 70% success rate in preventing influenza infection, though this varies by how well the vaccine strains match with the strains that actually wind up circulating during the season. The efficacy is higher against severe infections, hospitalizations, and deaths.
The vaccine is recommended for people considered to be at high risk. These include:
  • people over age 50
  • children age 6-59 months
  • children and adolescents who are receiving long-term aspirin therapy and therefore might be at risk for Reye Syndrome if they contract influenza
  • children and adults with heart disease and lung disease, including asthma
  • people who live in nursing homes or other institutional settings
  • people who have a chronic disease such as diabetes, anemia, or kidney disease
  • people who can transmit the flu to others at high risk. This group includes healthcare workers and employees at facilities caring for people at high risk.
  • women who will be pregnant during the flu season
  • people whose immune system is weakened because of chemotherapy
  • people with immune deficiency diseases including HIV or AIDS
About 25% of adults who receive the flu vaccine report mild soreness at the site of the injection. Young children may develop fever after a flu vaccine. There is also a slight risk of an allergic reaction to the vaccine.
Live-attenuated influenza vaccine (LAIV) is given by inhalation through the nose. It consists of live but weakened flu viruses - the same strains that are in the killed vaccine. LAIV has been shown to be very effective and safe and is now recommended for persons age 5-49.


How is the infection diagnosed?

A history and physical examination is the first step in diagnosing flu. Most of the time, the healthcare provider can make the diagnosis without further testing. In some cases, lab tests may be ordered, including:
  • a culture of samples of discharge taken from the nose or back of the throat to determine the type of virus
  • the cold agglutinins test, which measures germ-fighting chemicals that cause blood to clump at low temperatures
  • the influenza complement fixation test, which can detect the presence of disease-fighting chemicals made by the body
Getting a laboratory diagnosis of flu virus infections usually comes too late to help the infected individual, but is very useful for tracking the flu strains in the community. This information helps clinicians treat subsequent cases properly, and also provides guidance for the development of the next year's vaccine.

Long Term Effects

What are the long-term effects of the infection?

While most people recover from the flu without problems, others can develop complications such as:
  • pneumonia, an acute lung infection that can be caused by a virus or bacteria
  • acute bronchitis, an infection of the windpipe
  • secondary bacterial infections, which are infections caused by other germs besides those causing the flu infection
  • congestive heart failure
Influenza and its complications can cause serious disease or even death.

Other Risks

What are the risks to others?

The influenza virus is highly contagious. It is transmitted through respiratory secretions. Sneezing and coughing can spread these droplets. The virus can also be passed on when an individual with the flu touches his or her nose and then handles an object that another person later touches. The second person can then pick up the virus from the object and transfer it into his or her own respiratory tract by touching the face.


What are the treatments for the infection?

Flu is generally treated by addressing its symptoms. Bed rest and drinking plenty of fluids help prevent some complications. Antibiotics do not work against the viruses and can reduce the body's ability to fight viruses. Medications such as acetaminophen (i.e., Tylenol), ibuprofen (i.e., Advil, Motrin), or aspirin can help reduce fever and muscle aches.
However, several recent studies showed that people who used fever-fighting medications such as aspirin and acetaminophen had flu symptoms several days longer than those who did not use these medications.
In addition, aspirin (and aspirin-containing products) should never be given to children 17 years of age or younger with influenza as this may cause a rare but serious disease called Reye Syndrome.
There are currently 4 antiviral medications approved in the US and many other countries to prevent or treat flu. These include:
  • amantadine (i.e., Symmetrel) and rimantadine (i.e., Flumadine), which are effective only against the influenza A strain of the virus. Currently the Centers for Disease Control and Prevention has recommended that these antivirals not be used until further notice because currently circulating flu strains have developed resistance to them.
  • oseltamivir (i.e., Tamiflu), which is approved for both prevention and treatment of the influenza A and B viruses. Oseltamivir is a capsule taken by mouth which shortens the length of the illness by 1 to 1-1/2 days.
  • zanamivir (i.e., Relenza), which is approved - for treatment only - of both influenza A and B viruses. Zanamivir is inhaled through the mouth and reduces the length of illness by 1 to 1-1/2 days.
A number of natural medications (herbs, vitamins or supplements) have been studied in the treatment of influenza. The Natural Medicines Comprehensive Database rates Elderberry and N-Acetyl Cysteine as "possibly effective" while concluding that zinc and zinc lozenges are "possibly ineffective." They've determined that there is "insufficient evidence" at the present time for Andrographis, Echinacea, Panax ginseng and Oscillococcinum.
A decongestant called phenylpropanolamine (PPA), found in many over-the-counter cold remedies, has recently been linked to strokes in young women. The Food and Drug Administation (FDA) has requested that manufacturers stop producing medications containing PPA. In view of the FDA's public health advisory, anyone who has products at home containing phenylpropanolamine should discard them.

Side Effects

What are the side effects of the treatments?

Side effects of fever-fighting medications such as ibuprofen, aspirin, and acetaminophen include stomach upset and allergic reactions. Amantadine can cause anxiety, depression, and insomnia. Zanamivir can cause airway spasm and reduced airflow in people with chronic respiratory disease. Oseltamivir, amantadine, and rimantadine may cause nausea and vomiting.

After Treatment

What happens after treatment for the infection?

Flu symptoms go away in 7 to 10 days for most people. Some people can develop complications, however. The flu can cause pneumonia, even in healthy people.


How is the infection monitored?

A bout of flu usually clears up within 7 to 10 days. Any new or worsening symptoms should be reported to the healthcare provider.

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