Influenza is a viral infection that attacks the respiratory system, including your nose, throat, bronchial tubes and lungs. Influenza, commonly called the flu, is not the same as the stomach viruses that cause diarrhea and vomiting.

Anyone can get the flu, but young children, older adults, people with weakened immune systems and those with chronic illnesses are especially vulnerable. If you're at high risk of flu, your first line of defense is an annual flu shot.

Approximately 50 million people died worldwide in the 1918 influenza pandemic. Today, according to the Centers for Disease Control and Prevention, as many as 36,000 people in the United States die each year of complications of influenza and more than 200,000 are hospitalized.

Initially, the flu may seem like a common cold with a runny nose, sneezing and sore throat. But colds usually develop slowly, whereas the flu tends to come on suddenly. And although a cold can be a nuisance, you usually feel much worse with the flu.

Common signs and symptoms of the flu include:

• Fever over 101 F (38 C) in adults, and often as high as 103 to 105 F (39.5 C to 40.5 C) in children
• Chills and sweats
• Headache
• Dry cough
• Muscular aches and pains, especially in your back, arms and legs
• Fatigue and weakness
• Nasal congestion
• Loss of appetite
• Diarrhea and vomiting in children

When to see a doctor
If you have flu symptoms and are at risk of complications, see your doctor right away. Taking antiviral drugs within the first 48 hours after you first notice symptoms may reduce the length of your illness by a day or two and may help prevent more serious problems. Seek immediate medical care if you have signs and symptoms of pneumonia. These include a severe cough that brings up phlegm, a high fever and a sharp pain when you breathe deeply. If you have bacterial pneumonia, you'll need treatment with antibiotics.

Flu viruses travel through the air in droplets when someone with the infection coughs, sneezes or talks. You can inhale the droplets directly, or you can pick up the germs from an object, such as a telephone or computer keyboard, and then transfer them to your eyes, nose or mouth.

The flu is caused by three types (strains) of viruses — influenza A, B and C. Type A can be responsible for the deadly influenza pandemics (worldwide epidemics) that strike every 10 to 40 years. Type B can lead to smaller, more localized outbreaks. And either types A or B can cause the flu that circulates almost every winter. Type C has never been connected with a large epidemic.

Type C is a fairly stable virus, but types A and B are constantly changing, with new strains appearing regularly. Once you've had the flu, you develop antibodies to the strain that caused it, but those antibodies won't protect you from new strains. That's why doctors recommend getting a flu shot every year.

You're at increased risk of influenza or its complications if you:

• Are an infant or young child
• Are over age 50
• Are a resident of a nursing home or other long term care facility
• Have a chronic disorder, such as diabetes or heart, kidney or lung disease
• Have a weakened immune system, such as from medications or HIV infection
• Will be pregnant during flu season
• Work in a health care facility where you're more likely to be exposed to the flu virus
• Are in regular, close contact with infants or young children

Children on long-term aspirin therapy also may be at greater risk.

The flu vaccine is safe for children 6 months and older. If your child isn't at risk of the flu but lives with someone who is, you still may want to have your child vaccinated. That way, your child is less likely to infect others. The more people immunized, the less likely it is that the flu will spread through a community.

If you're young and healthy, influenza usually isn't serious. Although you may feel miserable while you have it, the flu usually goes away with no lasting effects. But high-risk children and adults may develop complications such as:

• Ear infections
• Acute sinusitis
• Bronchitis
• Pneumonia
• Encephalitis

Of these, pneumococcal pneumonia, a serious bacterial infection of the lungs, is the most common and most serious. For older adults and people with a chronic illness, pneumonia can be deadly. The best protection is vaccination against both pneumococcal pneumonia and influenza.

Usually, you'll need nothing more than bed rest and plenty of fluids to treat the flu. But in some cases, your doctor may prescribe an antiviral medication such as oseltamivir (Tamiflu) or zanamivir (Relenza).

These drugs, which treat both influenza A and B, work by deactivating an enzyme the virus needs to grow and spread. If taken soon after you notice symptoms, they may shorten your illness by a day or so. Oseltamivir is an oral medication, but zanamivir is inhaled through a device similar to an asthma inhaler and shouldn't be used by anyone with respiratory conditions, such as asthma and lung disease.

Both medications can cause side effects, including lightheadedness, nausea, vomiting, loss of appetite and trouble breathing. They can also lead to the development of antiviral-resistant viruses.

In November 2006, the Food and Drug Administration (FDA) required the maker of Tamiflu to include a warning that people with the flu, particularly children, may be at increased risk of self-injury and confusion after taking Tamiflu. The FDA recommends that individuals with the flu who take Tamiflu be closely monitored for signs of unusual behavior. Discuss possible side effects with your doctor before starting any antiviral medication.

If you do come down with the flu, these measures may help ease your symptoms:

• Drink plenty of liquids. Choose water, juice and warm soups to prevent dehydration. Drink enough so that your urine is clear or pale yellow.
• Rest. Get more sleep to help your immune system fight infection.
• Try chicken soup. It's not just good for your soul — it really can help relieve flu symptoms by breaking up congestion.
• Consider pain relievers. Use an over-the-counter pain reliever such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin, others) cautiously, as needed. Remember, pain relievers may make you more comfortable, but they won't make your symptoms go away any faster and may have side effects. Ibuprofen may cause stomach pain, bleeding and ulcers. If taken for a long period or in higher than recommended doses, acetaminophen can be toxic to your liver. Talk to your doctor before giving acetaminophen to children. And don't give aspirin to children or teens because of the risk of Reye's syndrome, a rare but potentially fatal disease.

These steps can help you stay healthy, even at the height of flu season:

• Get an annual flu vaccination. The best time to be vaccinated is October or November. This allows your body time to develop antibodies to the flu virus before peak flu season, which in the Northern Hemisphere is usually December through March. However, getting a flu shot later in the flu season may still protect you. It takes up to two weeks to build immunity following a flu shot.

  Keep in mind that the flu vaccine doesn't offer complete protection, especially for older adults, but it can reduce the risk and severity of illness. According to the Centers for Disease Control and Prevention (CDC), when the match between flu vaccine and circulating strains of flu virus is close, a flu shot is between 70 and 90 percent effective in warding off illness in healthy people under age 65. It is less effective in older adults. Health experts recommend vaccinations for people over 50 because the flu shot may reduce the risk of flu-related complications, hospitalizations and deaths.

  Healthy people ages 2 to 49 alternatively may receive the flu vaccine via a nasal spray called FluMist. It protects against the same strains of influenza that the flu shot does. Like the flu shot, FluMist is given every year. Because FluMist contains live, but weakened, flu viruses, it shouldn't be given to pregnant women, people with weakened immune systems and those with chronic illnesses. A study published in 2006 found that FluMist was only between 30 and 57 percent effective in preventing the flu in adults. However, another study in 2006 found that giving nasal spray vaccine to school-age children helped to reduce the spread of flu in the community. And in 2007, researchers compared the effectiveness of FluMist with the flu shot in children ages 6 months to 5 years. The children treated with FluMist experienced about half as many cases of flu as did those treated with the shot. However, FluMist increases the risk of wheezing in this age group — especially in those who already have asthma or recurrent wheezing and in all children under age 2. In 2007, the Food and Drug Administration (FDA) therefore approved FluMist for children older than 2 years who don't have asthma or recurrent wheezing. Consult with your doctor about which form of vaccination may be best for you or your child.

  You can get the flu vaccine from your doctor, at public health centers and many pharmacies. In some areas, flu vaccines are also available at senior or community centers and at supermarkets.

• Wash your hands. Thorough and frequent hand washing is the best way to prevent many common infections. Scrub your hands vigorously for at least 15 seconds, rinse well and turn off the faucet with a paper towel. Or use an alcohol-based hand gel containing at least 60 percent alcohol.
• Eat right, sleep tight. A poor diet and poor sleep both lower your immunity and make you more vulnerable to infections. A balanced diet that emphasizes fresh fruits and vegetables, whole grains, and small amounts of lean protein works best for most people. On the other hand, the amount of sleep needed for a healthy immune system varies from person to person. In general, adults seem to do best on seven to eight hours of sleep a night. Older children and teens need more rest — between nine and 10 hours every night.
• Exercise regularly. Regular cardiovascular exercise — walking, biking, aerobics — boosts your immune system. Exercise won't prevent infection, but if you do come down with the flu, you may have less severe symptoms and recover more quickly than do people who aren't as fit.
• Avoid crowds during flu season. Flu spreads easily wherever people congregate — in child care centers, schools, office buildings, auditoriums and public transportation. By avoiding crowds whenever possible during peak flu season, you reduce your chances of infection.