W. Paul Glezen, M.D.
Department of Molecular Virology and Microbiology
Baylor College of Medicine
Houston, Texas

What is influenza?

Influenza, commonly referred to as the "flu," is an acute, contagious respiratory infection. The first of the human respiratory viruses to be isolated and characterized, influenza viruses have been studied the most extensively and are the best understood. The term itself, "influenza," may have come from the Latin word influo, meaning "to flow in," perhaps indicating its airborne transmission, or it may be of Italian origin, relating to an "influence," such as the weather, or mystical astrologic causes.


What causes influenza?

Influenza is caused by strains of the orthomyxoviruses. The influenza viruses are comprised of three major types-A, B, and C-and multiple subtypes. Influenza A and B are the two types of influenza viruses that most often cause disease in humans. Influenza A and B viruses have been studied more extensively than influenza C viruses.


How does it cause disease?

Influenza is most prevalent in the winter and the spring. It occurs following close contact with a person who has the illness. Spread by discharges from the mouth and nose of an infected person, the virus is then inhaled and multiplies in the newly infected person. Influenza may occur on a sporadic basis, or it may occur as epidemic influenza (i.e., involving a large, regional population) or as pandemic influenza (i.e., involving a worldwide population).


Who gets influenza?

All persons may contract influenza; however, younger children (under 2 years), pregnant women, American Indians, Alaskan Natives and older adults (over 65 years) are the most susceptible to its effects. Persons at high risk for the complications associated with influenza include those with preexisting medical conditions, such as:

  • Asthma
  • Neurological and neurodevelopmental conditions
  • Chronic lung disease
  • Heart disease
  • Blood disorders (such as sickle cell disease)
  • Endocrine disorders (such as diabetes mellitus)
  • Kidney disorders
  • Liver disorders
  • Metabolic disorders (such as inherited metabolic disorders and mitochondrial disorders)
  • Weakened immune system due to disease or medication (such as people with HIV or AIDS, or cancer, or those on chronic steroids)
  • People younger than 19 years of age who are receiving long-term aspirin therapy
  • People who are morbidly obese (Body Mass Index [BMI] of 40 or greater)


What are the common findings?

In young children, the most common findings of type A influenza include its sudden onset and its associated symptoms of high fever, headache, lack of appetite, fatigue, chills, and muscle aches. Common respiratory findings include a cough, a runny nose, and a sore throat. Other symptoms may include abdominal pain, swollen lymph nodes in the neck area, nausea, vomiting, and diarrhea.

In older children and adolescents with type A influenza, the onset of the illness is abrupt, and is associated with high fever, flushed face, chills, headache, muscle aches, and fatigue.

In type B influenza, children often will have typical "flu-like" symptoms with fever; however, adults frequently will have only respiratory tract symptoms without significant fever.

Influenza C viruses cause illnesses similar to type A influenza; however, the severity of the disease is usually less, and the duration of it is shorter.


How is influenza diagnosed?

Infection with the influenza virus is diagnosed more accurately from groups of patients exhibiting the classic symptoms of influenza, rather than an individual patient. Epidemics occur each winter, and usually begin with a sudden increase in its appearance in the primary care facilities of school-age children with febrile (associated with fever) respiratory tract illnesses.

A diagnostic test called a "Rapid Flu Test" is now available in most physician's offices. Unfortunately, the reliability of these tests is variable and a person with a negative test may still have the flu. Your health care provider will often make the diagnosis of flu based on your symptoms and physical exam. In certain circumstances, your provider may decide to send a nasal swab to a specialized laboratory for a more definitive diagnosis.


How is influenza treated?

For types A and B influenza viruses, the illness usually resolves itself after several days; however, fatigue and persistent coughing can last for two or more weeks. Bed rest, adequate hydration with oral fluids, control of fever and muscle aches with acetaminophen, and maintenance of comfortable breathing with nasal decongestants and humidifiers are the best courses of treatment in uncomplicated cases of influenza. A persistent cough may be treated with cough suppressants.

Preventative administration of antibiotics should be discouraged. For complicated cases of influenza, a physician should evaluate the patient, and may recommend antibiotic treatment for possible secondary bacterial infections. The neuraminidase inhibitor oseltamivir (Tamiflu) is FDA-approved for the treatment of uncomplicated acute influenza in patients 1 year and older who have been symptomatic for no more than 2 days. The neuraminidase inhibitor zanamivir formulated for oral inhalation (Relenza) is FDA-approved for the treatment of influenza in patients 7 years of age and older who, similar to approved uses for oseltamivir, have uncomplicated illness and have been symptomatic for no more than 2 days.  These treatments have limited benefit and are recommended only for those HIGH RISK patients (see list above) or patients with severe symptoms.


What are the complications?

A patient's recovery from a case of uncomplicated influenza generally is considered to be excellent.

Complications that may occur as a result of influenza include bacterial infections of the respiratory tract (particularly pneumonia), acute otitis media (ear infections), and sinusitis. Acute myositis, (i.e., severe pain and tenderness in the calves of both legs that occurs suddenly, often with a refusal to walk) may also occur.

Reye's syndrome may occur as a result of influenza, most commonly when aspirin or aspirin-containing compounds are used in children with influenza. Reye's syndrome is a constellation of symptoms that can result in degeneration of the liver and/or swelling of the brain.

Rare complications of influenza include encephalitis and other neurologic illnesses (e.g., transverse myelitis, Guillain-Barr syndrome, Parkinson disease), cardiac inflammation (e.g., pericarditis, myocarditis), and kidney failure following myositis (acute inflammation of the muscle).

Despite improvements in living standards and the introduction of antibiotics, an average of 30,000 deaths still are attributed to influenza each year. Most deaths occur in those patients with preexisting chronic medical conditions involving the pulmonary or the cardiovascular systems, in very young patients (less than two years of age), or in elderly patients (older than 65 years of age).


How can influenza be prevented?

The influenza vaccine is the primary method for preventing influenza and its more severe complications. To be effective, the vaccine must contain antigens similar to those of the most likely current strain of the virus. In years when a new strain arises and causes widespread outbreaks, the available vaccine may contain a previous strain of the virus, which may give only modest protection from the flu.

Worth noting, the influenza vaccine does not affect the safety of mothers who are breastfeeding or their infants.

For those previously unvaccinated children who are 6 months to 8 years of age, two doses of the vaccine should be administered at least one month apart in order for it to be effective. For those children who are older than nine years, only one dose of the vaccine is necessary.

A live, attenuated (weakened) influenza virus vaccine (FluMist") administered by nasal spray is now available for healthy children over 2 years of age.  Recent studies have suggested that this flu nasal spray provides better protection to children and all children between 2 and 8 years of age should receive this nasal spray vaccine. If it is not available, these children should still receive a flu shot.

The live flu nasal spray should not be used in the following populations:

  • Persons aged <2 years or >49 years;
  • Those with contraindications listed in the package insert:
    • Children aged 2 through 17 years who are receiving aspirin or aspirin-containing products;
    • Persons who have experienced severe allergic reactions to the vaccine or any of its components, or to a previous dose of any influenza vaccine;
  • Pregnant women;
  • Immunosuppressed persons;
  • Persons with a history of egg allergy;
  • Children aged 2 through 4 years who have asthma or who have had a wheezing episode noted in the medical record within the past 12 months, or for whom parents report that a health care provider stated that they had wheezing or asthma within the last 12 months (Table [footnote]). [For those aged ≥5 years with asthma, recommendations are described in item 4 of this list];
  • Children with High Risk conditions (see list above) should consult their healthcare provider for studies are limited.
  • Children who are severely immunosuppressed
  • Persons who have taken influenza antiviral medications within the previous 48 hours.

Side effects to the vaccine may occur, and they include fever, "flu-like" symptoms of fatigue and muscle aches, and tenderness at the site of the inoculation (if given by injection). The occurrence of febrile convulsions, which have been associated with the vaccine in very young patients, is rare, and studies have shown no association of an increased frequency of Guillain-Barr syndrome and the influenza vaccine.


Can the influenza vaccine prevent acute otitis media?

Acute otitis media (i.e., ear infection) is the most common cause for illness visits to the pediatrician in the United States, most often occurring in children between the ages of 6 months and 3 years, with the highest incidence in the 6- to 12-month age group.

Studies suggest that the influenza vaccine can decrease the incidence of acute otitis media in children, especially those children between the ages of 6 and 30 months, during the influenza season. These same studies also suggest that other vaccines against respiratory viruses may be an effective way to reduce the incidence of acute otitis media in children.


What research is being done?

Whereas the currently available antiviral drugs, oseltamivir and zanamivir, are effective against influenza A and B viruses, recent resistance has been reported. In intravenous medication, panamivir has been approved for administration to severely ill hospitalized patients with influenza.


Links to other information

Information regarding influenza is available through the Centers for Disease Control and Prevention (CDC) Web site at CDC FLU FACTS.

State and local health departments can be contacted for information regarding the availability of the influenza vaccine, access to vaccination programs, and information about state or local influenza activity.



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About the Author

Dr. Glezen is professor of microbiology and pediatrics at Baylor College of Medicine in Houston, Texas. His research has focused on the consequences and the prevention of respiratory viruses in children.

Dr. Glezen has published more than 125 papers and chapters related to his research. His three grandchildren, Claire, Tyler, and Meghan Gahm, have flourished under the pediatric care of Dr. Dan Feiten.

Reviewed 11/3/2010

Revised 11/3/2010

Copyright 2012 W. Paul Glezen, M.D., All Rights Reserved