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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Man to Man Man to Man

Influenza -- Just the Flu?

September 11, 2013

By Harvey B. Simon M.D.

Harvard Medical School

It happens every year. The days grow shorter, the temperature drops, footballs fly — and the flu strikes. Influenza is so common that it's easy to dismiss this seasonal woe as "just a virus" or "just the flu."

It's true that the flu is caused by a virus and that most patients recover without specific therapy. But it's also true that thousands of Americans die from the flu each year and millions are sick enough to miss work or school. Influenza is a serious infection but it can be prevented and treated.

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Meet the Influenza Viruses

A large group of viruses belong to the influenza family. Many infect animals, ranging from pigs and horses to poultry and other birds. Most animal flu viruses stay where they belong, but scientists worry about the bird flu virus because it is so serious when it infects humans.

Influenza Epidemics

Influenza is a world-wide problem. Because new strains of the virus crop up every year, new outbreaks occur annually. In most years, the new strains turn up first in Asia and then spread around the world to the U.S. That gives scientists a chance to identify epidemic strains and produce new vaccines before the flu bugs hit home.

The flu season in the United States runs from about Thanksgiving to Easter, with most cases occurring in the dead of winter. In a typical year, up to 10% of us get the flu, over 200,000 people are sick enough to require hospitalization and over 20,000 Americans die from the infection. And the toll can double during epidemics, which occur every 10 to 15 years.

Nearly all human flu infections are caused by human strains of the influenza A or B viruses. Influenza A is the more serious virus. It's an expert at disguising itself by changing the proteins on its outer coat. Because of that, people who are immune to an old strain of the flu virus are not protected against new strains. That's why you need to get a new flu shot every year. Usually, one flu strain becomes the dominant one in any given year.

Influenza is highly contagious. As a respiratory virus, it spreads on tiny droplets of mucus that spew into the air when you cough, sneeze or simply exhale. People close at hand are the most likely to catch the flu. That's why the infection spreads so quickly though families, health care facilities and other places where people live close to each other. The virus can also be spread by hand-to-hand contact.

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Influenza hits fast. After an incubation period of just one to two days, the symptoms start abruptly. Most patients are feverish and high temperatures between 103° and 104° are common. Nearly everyone has a runny nose and sore throat. But unlike ordinary colds, the flu also produces a hacking, dry cough. Muscle and joint aches can be severe. Headache, burning eyes, weakness and extreme fatigue add to the misery. In most cases, the high fever and severe distress settle down in two to five days, but the cough can linger for a week or two and the fatigue even longer.

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In general, older children and young adults usually handle the flu very well compared to infants, pregnant women and seniors.

The most serious complication is pneumonia. Seniors, people with chronic illnesses and young children are at greatest risk. They need flu vaccinations and medical treatments the most. (Many children need two shots.)

In some cases, pneumonia is caused by the flu virus itself. It's a particularly deadly problem that begins early in the infection and progresses rapidly, with a severe dry cough and shortness of breath. Bacterial pneumonia is more common but more treatable. It starts later, after patients seem to be on the mend. The fever returns, the cough increases and patients produce or bring up or cough up thick, pussy sputum (phlegm).

Other flu complications can include asthma attacks, ear infections, bronchitis, sinusitis, inflammation of the heart or other muscles and inflammation of the nervous system.

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Most cases of the flu are diagnosed because the symptoms are easily recognized, they appear during flu season and the virus is going around the community. But milder cases of the flu can resemble other respiratory infections that also strike in the winter.

Doctors usually rely on clinical findings to diagnose the flu. But if the symptoms are unusual or very severe, or if the flu has not yet turned up in the community, special tests can confirm the diagnosis. Doctors use a rapid test that detects within minutes proteins from the influenza virus on a sample of the patient's nasal secretions. Blood tests, chest x-rays and other studies may be needed if complications develop.

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Whether or not you've had a flu shot this year, a few simple precautions can help protect you and your family:


  • Wash your hands. Alcohol-based hand rubs and gels are best. Look for one containing 60% to 95% isopropanol or ethanol. Ordinary soap and water will also help, but it's not necessary to use very hot water or "antibacterial" soaps. Antimicrobial towelettes may be useful when you're on the move. Wash carefully after any contact with folks who have flulike symptoms.


  • Keep your distance. The flu is most contagious within three feet of a patient. If your community is hard-hit this winter, try to minimize the time you spend in crowded places.


  • Wear a mask, particularly if you are at high risk for flu complications and you can't avoid getting close to flu patients. Be sure your mask fits well. Keep it dry and free of saliva, and change it periodically. (N-95 respirator masks are best.)


  • Protect others. Don't go to work if you have the flu. Use a tissue to cover your mouth when you sneeze or cough, and dispose of it properly. Wear a mask if you have to go out in public, especially in health care facilities.


New vaccines are produced for every flu season; each protects against the two strains of influenza A and single strain of influenza B that are heading our way in the fall. In the United States, October and November are the ideal months to get the vaccine. Men need one dose, but children ages 6 months to 8 years who have never been immunized need two doses.

Flu vaccine can be given as a shot or in the form of a nasal spray (FluMist). The nasal spray can be used only by healthy, non-pregnant individuals ages 2 to 49 years.

The injectable vaccine can be given to nearly everyone. New vaccines are now available that are not made from eggs. So even people with an egg allergy can get the shot. Side effects are mild and uncommon, and include a slightly sore arm or slight fever.

Immunization can reduce your risk of catching the flu by up to 80%. That's a big benefit, but nearly half the people who need protection the most don't get it.

The U.S. Centers for Disease Control and Prevention now recommend that everyone older than 6 months of age get flu vaccine, except in rare circumstances.

In case of a flu vaccine shortage, priority will be given to:

  • People with asthma, diabetes, or chronic diseases of their lungs, heart, blood, kidneys, liver
  • People who have illnesses or take medications that impair the immune system
  • Residents of chronic care facilities
  • Health care personnel and child care providers
  • Adults 50 and older
  • Caregivers and others who come into contact with high- risk people
  • Children ages 6 months to just before their 19th birthday (especially if they receive long-term aspirin therapy)
  • Pregnant women and women who are likely to be pregnant during the flu season

Medications for Prevention and Treatment

Antibiotics don't work against viruses, including influenza. But while there are no medications for ordinary viruses, special prescription drugs can be used to treat or prevent the flu. Two older drugs, amantadine and rimantadine, have lost their effectiveness. Two newer drugs, oseltamivir (TamiFlu) and zanamivir (Relenza), remain active against most strains of influenza A and B. But last flu season, there was a worrisome increase in resistance to oseltamivir.

Both of the newer medications target a viral enzyme called neuraminidase. Neither will cure the flu, but they can ease and shorten the illness if started within the first 24 to 36 hours of symptoms. Both drugs can also prevent influenza in unvaccinated persons who are exposed to the infection.

Zanamivir is given by inhalation from a nebulizer. It is approved for prevention in people ages 5 and older and for treatment in people ages 7 and older. Side effects may include wheezing, nausea, and vomiting.

Oseltamivir is available in tablet form. It is approved for prevention and treatment in patients older than one year. Side effects may include nausea and vomiting.

If you get the flu, ask your doctor if these medications are right for you. With or without an antiviral drug, be sure to get lots of rest and drink plenty of fluids. Acetaminophen can help ease fever and aches; aspirin is also effective, but should never be used by flu patients under age 18. Be sure to contact your doctor promptly if you think you're developing pneumonia or other complications that may require antibiotics or hospitalization.

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Respect the Flu

Although influenza is a serious infection, it doesn't get the respect it deserves. Don't make the mistake of dismissing the flu as "just a virus." Instead, talk to your doctor about an immunization each fall and take simple precautions to protect yourself and your family during influenza outbreaks. And if you get the flu, ask your doctor if medication might speed your recovery.

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Harvey B. Simon, M.D. is an Associate Professor of Medicine at Harvard Medical School and a member of the Health Sciences Technology Faculty at Massachusetts Institute of Technology. He is the founding editor of the Harvard Men's Health Watch newsletter and author of six consumer health books, including The Harvard Medical School Guide to Men's Health (Simon and Schuster, 2002) and The No Sweat Exercise Plan, Lose Weight, Get Healthy and Live Longer (McGraw-Hill, 2006). Dr. Simon practices at the Massachusetts General Hospital; he received the London Prize for Excellence in Teaching from Harvard and MIT.

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