Usually referred to as the flu or grippe, influenza is a highly infectious respiratory disease. The disease is caused by certain strains of the influenza virus. When the virus is inhaled, it attacks cells in the upper respiratory tract, causing typical flu symptoms such as fatigue, fever and chills, a hacking cough , and body aches. Influenza victims are also susceptible to potentially life-threatening secondary infections. Although the stomach or intestinal "flu" is commonly blamed for stomach upsets and diarrhea , the influenza virus rarely causes gastrointestinal symptoms. Such symptoms are most likely due to other organisms such as rotavirus, Salmonella , Shigella , or Escherichia coli .


The flu is considerably more debilitating than the common cold . Influenza outbreaks occur suddenly, and infection spreads rapidly. In the 1918–19 Spanish flu pandemic, the death toll reached a staggering 20 to 40 million worldwide. Approximately 500,000 of these fatalities occurred in the United States.

Influenza outbreaks occur on a regular basis. The most serious outbreaks are pandemics, which affect millions of people worldwide and last for several months. The 1918–19 influenza outbreak serves as the primary example of an influenza pandemic. Pandemics also occurred in 1957 and 1968 with the Asian flu and Hong Kong flu, respectively. The Asian flu was responsible for 70,000 deaths in the United States, while the Hong Kong flu killed 34,000.

Epidemics are widespread regional outbreaks that occur every two to three years and affect 5–10 percent of the population. The Russian flu in the winter of 1977 is an example of an epidemic. A regional epidemic is shorter lived than a pandemic, lasting only several weeks. Finally, there are smaller outbreaks each winter that are confined to specific locales.

The earliest existing descriptions of influenza were written nearly 2,500 years ago by the ancient Greek physician Hippocrates. Historically, influenza was ascribed to a number of different agents, including "bad air" and several different bacteria. In fact, its name comes from the Italian word for "influence," because people in eighteenth-century Europe thought that the disease was caused by the influence of bad weather. It was not until 1933 that the causative agent was identified as a virus.

There are three types of influenza viruses, identified as A, B, and C. Influenza A can infect a range of animal species, including humans, pigs, horses, and birds, but only humans are infected by types B and C. Influenza A is responsible for most flu cases, while infection with types B and C virus are less common and cause a milder illness.


The annual death toll attributable to influenza and its complications averages 20,000 in the United States alone. In the United States, 90 percent of all deaths from influenza occur among persons older than 65. Flu-related deaths have increased substantially in the United States since the 1970s, largely because of the aging of the American population. In addition, elderly persons are vulnerable because they are often reluctant to be vaccinated against flu.

Hospitalization due to complications of influenza are common in children. Among children with chronic illnesses, about 500 children per every 100,000 between the ages of birth and age four are hospitalized annually due to influenza, while about 100 children per 100,000 without chronic illnesses are hospitalized annually. Among those with underlying high-risk conditions, infants younger than six months have the highest hospitalization rates (approximately 10–40 per 100,000 population).

Causes and symptoms

Approximately one to four days after infection with the influenza virus, the victim is hit with an array of symptoms. "Hit" is an appropriate term, because symptoms are sudden, harsh, and unmistakable. Typical influenza symptoms include the abrupt onset of a headache , dry cough, and chills, rapidly followed by overall achiness and a fever that may run as high as 104°F (40°C). As the fever subsides, nasal congestion and a sore throat become noticeable. Flu victims feel extremely tired and weak and may not return to their normal energy levels for several days or even a couple of weeks.

Influenza complications usually arise from bacterial infections of the lower respiratory tract. Signs of a secondary respiratory infection often appear just as the victim seems to be recovering. These signs include high fever, intense chills, chest pains associated with breathing, and a productive cough with thick yellowish green sputum. If these symptoms appear, medical treatment is necessary. Other secondary infections, such as sinus or ear infections may also require medical intervention. Children with heart and lung problems, as well as other chronic diseases, are at higher risk for complications from influenza.

With children and teenagers, it is advisable to be alert for symptoms of Reye's syndrome , a rare, but serious complication. Symptoms of Reye's syndrome are nausea and vomiting , and more seriously, neurological problems such as confusion or delirium. The syndrome has been associated with the use of aspirin to relieve flu symptoms.


Although there are specific viral culture tests available to identify the flu virus strain from respiratory samples, results can take several days. Therefore, doctors typically rely on a set of symptoms and the presence of influenza in the community for diagnosis. Specific tests are useful to determine the type of flu in the community, but they do little for individual treatment. Doctors may administer tests, such as throat cultures, to identify secondary infections.

Several rapid (30-minute) diagnostic tests for flu have become commercially available. These tests appear to be especially useful in diagnosing flu in children, allowing doctors to make more accurate treatment decisions in less time.


Essentially, a bout of influenza must be allowed to run its course. Symptoms can be relieved with bed rest and by keeping well hydrated. A steam vaporizer may make breathing easier, and pain relievers can mask the aches and pain. Food may not seem very appetizing, but an effort should be made to consume nourishing food. Recovery should not be pushed. Returning to normal activities too quickly invites a possible relapse or complications.


Since influenza is a viral infection, antibiotics are useless in treating it. However, antibiotics are frequently used to treat secondary infections.

A transmission electron microscopy (TEM) image of influenza viruses budding from the surface of an infected cell. (CNRI/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc.)
A transmission electron microscopy (TEM) image of influenza viruses budding from the surface of an infected cell.
(CNRI/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc.)

Over-the-counter medications are used to treat flu symptoms, but it is not necessary to purchase a medication marketed specifically for flu symptoms. Any medication that is designed to relieve symptoms, such as pain and coughing, will provide some relief. Medications containing alcohol, however, should be avoided because of the dehydrating effects of alcohol. The best medicine for symptoms is simply an analgesic, such as acetaminophen or naproxen. (Without a doctor's approval, aspirin is generally not recommended for people under 18 owing to its association with Reye's syndrome, a rare aspirin-associated complication seen in children recovering from the flu. To be on the safe side, children should receive acetaminophen or ibuprofen to treat their symptoms.)

As of 2004, there were a number of antiviral drugs marketed for treating influenza. To be effective, treatment should begin no later than two days after symptoms appear. These medications are useful for decreasing the severity and duration of symptoms. Antivirals may be useful in treating patients who have weakened immune systems or who are at risk for developing serious complications. They include amantadine (Symmetrel, Symadine) and rimantadine (Flumandine), which work against Type A influenza, and zanamavir (Relenza) and oseltamavir phosphate (Tamiflu), which work against both Types A and B influenza. Amantadine and rimantadine can cause side effects such as nervousness, anxiety , lightheadedness, and nausea . Severe side effects include seizures, delirium, and hallucination, but are rare and are nearly always limited to people who have kidney problems, seizure disorders, or psychiatric disorders. Zanamavir and oseltamavir phosphate can cause dizziness , jitters, and insomnia.


Following proper treatment guidelines, healthy people under the age of 65 usually suffer no long-term consequences associated with flu infection. The elderly and the chronically ill are at greater risk for secondary infection and other complications, but they can also enjoy a complete recovery.

Most people recover fully from an influenza infection, but it should not be viewed complacently. Influenza is a serious disease, and approximately one in 1,000 cases proves fatal.


The Centers for Disease Control and Prevention recommends that people get an influenza vaccine injection each year before flu season starts. In the United States, flu season typically runs from late December to early March. Vaccines should be received two to six weeks prior to the onset of flu season to allow the body enough time to establish immunity. Adults only need one dose of the yearly vaccine, but children under nine years of age who have not previously been immunized should receive two doses with a month between each dose.

Each season's flu vaccine contains three virus strains that are the most likely to be encountered in the coming flu season. When there is a good match between the anticipated flu strains and the strains used in the vaccine, the vaccine is 70–90 percent effective in people under 65. Because immune response diminishes somewhat with age, people over 65 may not receive the same level of protection from the vaccine, but even if they do contract the flu, the vaccine diminishes the severity and helps prevent complications.

The virus strains used to make the vaccine are inactivated and will not cause the flu. In the second half of the twentieth century, flu symptoms were associated with vaccine preparations that were not as highly purified as modern vaccines, not to the virus itself. In 1976, there was a slightly increased risk of developing Guillain-Barré syndrome, a very rare disorder, associated with the swine flu vaccine. This association occurred only with the 1976 swine flu vaccine preparation and as of 2004 had not recurred.

Serious side effects with modern vaccines are extremely unusual. Some people experience a slight soreness at the point of injection, which resolves within a day or two. People who have never been exposed to influenza, particularly children, may experience one to two days of a slight fever, tiredness, and muscle aches. These symptoms start within six to 12 hours after the vaccination .

It should be noted that certain people should not receive an influenza vaccine. Infants six months and younger have immature immune systems and will not benefit from the vaccine. Since the vaccines are prepared using hen eggs, people who have severe allergies to eggs or other vaccine components should not receive the influenza vaccine. As an alternative, they may receive a course of amantadine or rimantadine, which are also used as a protective measure against influenza. Other people who might receive these drugs are those that have been immunized after the flu season has started or who are immunocompromised, such as people with advanced HIV disease. Amantadine and rimantadine are 70–90 percent effective in preventing influenza.

Certain groups are strongly advised to be vaccinated because they are at increased risk for influenza-related complications. These groups are:


Common cold —A mild illness caused by upper respiratory viruses. Usual symptoms include nasal congestion, coughing, sneezing, throat irritation, and a low-grade fever.

Epidemic —Refers to a situation in which a particular disease rapidly spreads among many people in the same geographical region in a relatively short period of time.

Guillain-Barré syndrome —Progressive and usually reversible paralysis or weakness of multiple muscles usually starting in the lower extremities and often ascending to the muscles involved in respiration. The syndrome is due to inflammation and loss of the myelin covering of the nerve fibers, often associated with an acute infection. Also called acute idiopathic polyneuritis.

Pandemic —A disease that occurs throughout a regional group, the population of a country, or the world.

  • children under age two
  • all people 65 years and older
  • residents of nursing homes and chronic-care facilities, regardless of age
  • adults and children who have chronic heart or lung problems, such as asthma
  • adults and children who have chronic metabolic diseases, such as diabetes and renal dysfunction, as well as severe anemia or inherited hemoglobin disorders
  • children and teenagers who are on long-term aspirin therapy
  • women who will be in their second or third trimester during flu season or women who are nursing
  • anyone who is immunocompromised, including HIV-infected persons, cancer patients, organ transplant recipients, and patients receiving steroids, and those receiving chemotherapy or radiation therapy
  • anyone in contact with the above groups, such as teachers, care givers, healthcare personnel, and family members
  • travelers to foreign countries

A person need not be in one of the at-risk categories listed above, however, to receive a flu vaccination. Anyone who wants to forego the discomfort and inconvenience of an influenza attack may receive the vaccine.

Parental concerns

Parents should make sure that their children who fall into any of the risk categories should be vaccinated against the flu. Pregnant women in the second or third trimesters should also be vaccinated. Flu vaccines are available through pediatricians or local public health departments. Parents should also make sure kids follow good hygiene practices, including regular hand washing, and covering the mouth when sneezing or coughing. Children may acquire secondary infections, such as ear infections or sinus infections, so parents should call the pediatrician if a child develops a high fever, sudden pain in the ears or sinuses, or develops a productive cough with thick yellow-green phlegm.



Subbarao, Kanta. "Influenza Viruses." In Principles and Practice of Pediatric Infectious Diseases , 2nd ed. Edited by Sarah S. Long et al. St. Louis, MO: Elsevier, 2003.

Wright, Peter. "Influenza Viruses." In Nelson Textbook of Pediatrics. Edited by Richard E. Behrman et al. Philadelphia: Saunders, 2004.


Larkin, M. "Will influenza be the next bioweapon?" Lancet Infectious Disease 138 (January 7, 2003): 53.

Neff, M. J. "ACIP Releases 2004 Guidelines on the Prevention and Control of Influenza." Annals of Internal Medicine 70) (July 2004): 199–204.

Stiver, G. "The treatment of influenza with antiviral drugs." Canadian Medical Association Journal 138 (January 2003): 49–56.


Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. Web site:

National Institute of Allergy and Infectious Diseases (NIAID). 31 Center Drive, MSC 2520, Bethesda, MD 20892–2520. Web site:


"Flu." Health Matters , November 2004. Available online at (accessed December 28, 2004).

Julia Barrett Rebecca J. Frey, PhD Rosalyn Carson-DeWitt, MD

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