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 affliction 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.

Up close and personal: Meet the flu bug

A large group of viruses belong to the influenza family. Many target animals, ranging from pigs and horses to poultry and other birds. Most animal flu viruses stay where they belong, but the bird flu virus can spread to humans. Scientists worry about this virus because it is so serious when it infects humans (see box).

Nearly all human infections are caused by human strains of the influenza A or B virus. Influenza A is the more serious. It has mastered the nasty trick of 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 each year.

Bird flu Many animals get the flu, but each has its own type of flu virus. When people get the flu, it's almost always caused by a human flu virus. Bird flu, also called avian flu, is caused by strains of influenza virus that normally infect birds. Some animal flu viruses develop mutations that allow them to infect humans. World health authorities are particularly concerned about one bird flu virus, called avian influenza A (H5N1), which has infected millions of birds. It began in Asia in the 1990s and has been carried by migrating birds to many parts of Europe and Africa. No cases have occurred in the Americas. Bird flu is very contagious for birds, but does not easily spread to humans. Only a few hundred human cases have been diagnosed, mostly in people who got the virus from sick birds. But when bird flu strikes humans, it is very serious; about half the patients have died. Scientists are trying to control bird flu by containing the infection in birds, and by reducing contact between birds and people in areas with the virus. But they worry that if the virus changes to allow rapid person-to-person spread, it could produce a deadly human epidemic similar to the flu pandemic of 1918. An avian flu vaccine is available for poultry. A bird flu vaccine for humans has been developed but is not yet available for clinical use. If the bird flu virus begins to spread in humans, control will depend on isolating sick patients, using good masks such as N95 respirators, and encouraging careful hygiene, including hand washing with alcohol-based rubs. Oseltamivir is the medication with the greatest promise against bird flu.

Influenza epidemics

Influenza is a worldwide problem. Because different 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 American scientists a chance to spot epidemic strains and produce new vaccines before the flu bug hits home.

In the U.S., the flu season runs roughly from 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 about 36,000 Americans die from the infection. And the toll can double during epidemics, which occur every 10 to 15 years.

"Just the flu?"

Spreading problems

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, which is why the infection spreads so quickly through families, health care facilities, and other places where people live or work close to each other. The virus can also be spread by hand-to-hand contact.

Why winter? The flu loves winter. In the northern hemisphere, it comes around between November and March, but in the southern hemisphere, it hits from May to September, the coldest months. In the tropics, however, there is no true flu season — and very little flu. Many respiratory infections peak in winter, when people cluster together indoors. But for years, scientists have wondered if there is something special about the influenza virus that accounts for its striking seasonality. And a 2007 study found an answer. Using guinea pigs infected with human flu viruses, scientists in New York found that the virus is transmitted much more efficiently in a cool environment. Animals who were housed at 41° F shed the virus nearly two days longer than animals housed at 68° F, and a temperature of 86° F blocked transmission of the virus altogether. Low humidity provided another boost for the bug; the virus spread much more readily at 20% humidity than at 80%. Next summer, before you complain about the heat and humidity, remember that it's protecting you from the flu.

Symptoms

Influenza hits fast. After an incubation period of just one to two days, the symptoms start abruptly. Most patients are feverish, and high temperatures in the 103° F to 104° F range 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.

Complications

The most serious — and deadly — complication is pneumonia. Young children, senior citizens, and people with chronic illnesses are at greatest risk, which is why they have the greatest need for preventive vaccinations and medical treatments.

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 raise 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.

"Just the flu?"

Is it the flu?
	Allergy	Cold	Sinusitis	Flu
Symptom
Sneezing	Yes	Yes	No	No
Itching eyes or throat	Yes	No	No	No
Nasal discharge	Watery	Watery	Thick, discolored	Thin
Bad breath or taste in mouth	No	No	Yes	No
Facial pain/pressure	No	Mild	Yes	No
Fever	No	Low grade	Low to moderate	High
Cough	No	Mild	Mild	Severe
Muscle aches	No	Mild	Mild	Severe
Headache	No	Mild	Mild	Severe
Fatigue, weakness	No	Mild	Mild	Severe
Treatment
Fluids	No	Yes	Yes	Yes
Inhaled steam	No	No	Yes	No
Antihistamines	Yes	Yes	No	No
Decongestants	Yes	Yes	Yes	Sometimes
Antibiotics	No	No	Yes	No
Antivirals	No	No	No	Sometimes

Diagnosis

Most cases are diagnosed because the season is right, the virus is going around the community, and the symptoms are typical. But milder cases of the flu can resemble other respiratory infections that also strike in the winter. You can use the table above to see if your symptoms are likely to be the flu or a less serious problem — and to start thinking about what to do. And remember to consult your doctor for personal diagnosis and treatment.

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. Most often, labs use a rapid test that can detect proteins from the influenza virus on a patient's nasal swab within minutes. Blood tests, chest x-rays, and other studies may be needed if complications develop.

Prevention: Hygiene

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, and 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 flu-like 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 if you are in a high-risk group and you can't avoid getting up-close and personal with possible flu victims. Be sure your mask fits well. Keep it free of saliva and dry, and change it periodically. N95 respirator masks are best; they're recommended to protect people at high risk for flu complications, and they're essential for possible exposures to bird flu.

• Protect others. Don't go to work or school 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 public, especially in health care facilities.

Prevention: Vaccination

New vaccines are produced for every flu season; each protects against the two strains of influenza A and one strain of influenza B that are most likely heading our way in the fall. In the U.S., October and November are the ideal months to get the vaccine. Children ages six months to eight years who have never been immunized need two doses, but one dose will suffice for all others.

Two types of flu vaccine are available. The nasal spray can be used only by healthy, non-pregnant individuals ages 2 to 49. The injectable vaccine can be given to nearly everyone, except people who are allergic to eggs or to the vaccine itself. Side effects are mild and uncommon, amounting to a slightly sore arm or a 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. If vaccine supplies are adequate, everyone above six months of age should get a flu vaccine this fall. Here is a list of high-priority vaccine candidates:

• All children ages six months to four years

• All adults age 50 and older

• Children and adolescents ages 6 months to 18 years who receive long-term aspirin therapy

• Women who are likely to be pregnant during the flu season

• People who have asthma, diabetes, or chronic diseases of their lungs, heart, blood, kidneys, or 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

• Caregivers and household contacts of persons with medical conditions that put them at risk

Flu shots: Unexpected benefits Protection from the flu is reason enough to get a flu shot every fall. But there's even more. A major study of 286,383 people age 65 and above found that flu vaccinations were associated with a 19% to 23% reduction in the risk of hospitalizations for heart disease and stroke, along with a 29% to 32% reduction in the risk of hospitalization for influenza or pneumonia. All in all, senior citizens who got flu shots had a nearly 50% reduction in the risk of death during the winter flu season.

Prevention and treatment: Medications

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 (Symmetrel) and rimantadine (Flumadine), have lost their effectiveness, but two new drugs remain active against most strains of influenza A and B. Both 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 flu symptoms. Both drugs can also be used to prevent influenza in unvaccinated persons who are exposed to the infection.

Zanamivir (Relenza) is administered by inhalation from a nebulizer. It is approved for prevention in people age five and older and for treatment in people age seven and older. Side effects may include wheezing, nausea, and vomiting; behavioral abnormalities have also been reported.

Oseltamivir (Tamiflu) is available in tablet form. It is approved for prevention and treatment in patients above one year of age. Side effects may include nausea and vomiting; behavioral abnormalities have also been reported.

If you get the flu and can start treatment within about 36 hours, ask your doctor about oseltamivir or zanamivir. With or without an antiviral drug, be sure to get lots of rest and drink plenty of fluids. Acetaminophen (Tylenol and other brands) can help ease fever and aches; aspirin is also effective, but should never be used by flu patients under 18 years of age. And be sure to contact your doctor promptly if you think you're developing pneumonia or other complications that may require antibiotics or hospitalization.

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, get 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.