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Millions of people, including 500,000 Americans, died in the 1918 influenza pandemic. Today, according to the Centers for Disease Control and Prevention, as many as 36,000 Americans die each year of complications of influenza and more than 200,000 are hospitalized.

Influenza is a viral infection that attacks the respiratory system, including your nose, throat, bronchial tubes and lungs. Although it's commonly called the flu, influenza is not the same as the stomach virus that causes 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. Although the shot doesn't offer 100 percent protection, it can reduce your chance of infection and help prevent serious complications if you do get sick.

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. Children with the flu tend to have higher fevers than adults have — often as high as 103 to 105 F.
• 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. Although children may have these signs, diarrhea and vomiting are rare in adults.

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 is responsible for the deadly influenza pandemics (worldwide epidemics) that strike every 10 to 40 years. Type B can lead to smaller, more localized outbreaks that generally occur every three to 15 years. And either types A or B can cause the flu that circulates almost every winter. Type C is less common and causes only mild symptoms.

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 virus that caused it, but those antibodies won't protect you from new strains. That's why doctors recommend getting a flu shot every year.

Infants and young children and people older than 65 are considered at high risk of complications of influenza. You're also at increased risk of influenza or its complications if you:

• 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 from medications or HIV infection
• Will be in your second or third trimester of pregnancy 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 are at greater risk if they:

• Have asthma or another chronic lung condition
• Have cardiovascular disease
• Are taking medications to suppress their immune system
• Have sickle cell anemia, HIV infection, diabetes, chronic kidney disease, chronic metabolic diseases or are on long-term aspirin therapy

The flu vaccine is safe for children 6 months or 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 who are immunized, the less likely it is that the flu will spread through a community.

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 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.

If you're young and healthy, influenza usually isn't serious. Although you may feel completely 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 and pneumonia.

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.

Most often, 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, loss of appetite and trouble breathing. They can also lead to the development of antiviral-resistant viruses. Discuss the possible side effects with your doctor before starting any antiviral medication.

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

• Get an annual flu shot. 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. Keep in mind that the flu shot doesn't offer complete protection, especially for older adults, but it can reduce the risk of illness. In any given year, the flu shot may be 70 percent to 90 percent effective in young, otherwise healthy people but far less effective in older adults. Health experts still recommend vaccinations for people over 65 because the flu shot may reduce the risk of flu-related complications, hospitalizations and death.

  Healthy people ages 5 to 49 may receive the flu vaccine through 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 flu viruses, it shouldn't be used by pregnant women, people with weakened immune systems and those with chronic illnesses. 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 handwashing 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. Hand gels are easy to use and may be even more effective at killing germs than soap and water are.
• 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 every 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.
• Limit air travel. Researchers say that the pronounced drop in air travel that occurred in the fall of 2001 slowed the spread of the flu and delayed the start of flu season by several weeks. Ordinarily, new flu strains spread rapidly in November, during the height of the holiday travel season, so you may be able to reduce your risk by limiting your time in the air in the late fall.
• Avoid crowds during flu season. Flu spreads easily wherever people congregate — in child care centers, schools, large office buildings, auditoriums, even cruise ships. By avoiding crowds whenever possible during peak flu season, you reduce your chances of infection.

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 up. 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, acetaminophen can be toxic to your liver — even at recommended levels. Talk to your doctor before giving acetaminophen to children — it can cause liver and kidney damage. And don't give aspirin to children under age 16 because of the risk of Reye's syndrome, a rare but potentially fatal disease.