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As concerns about emerging infectious diseases fill the headlines, should you still worry about the plain old flu? Yes, according to health officials. Influenza is the viral infection that sickens millions of people each year and has the potential to cause serious complications, especially in children and seniors.

Here are the answers to common questions about the flu and flu vaccine.

A flu shot is between 70 percent and 90 percent effective in warding off illness, depending on the length and intensity of a given flu season and your overall health. In a few cases, people who get a flu shot may still get the flu, but they'll get a much less virulent form of the illness and, most important, they'll have a decreased risk of flu-related complications — especially pneumonia, heart attack, stroke and death — to which older adults are especially vulnerable.

You need an annual flu shot because the vaccine changes from year to year. The flu vaccine you got last year wasn't designed to fight the virus strains in circulation this season, or next.

Influenza viruses mutate so quickly that they can render one season's vaccine ineffective by the next season. A Centers for Disease Control and Prevention (CDC) advisory committee meets early in the year to estimate which strains of influenza virus will be most prevalent during the upcoming flu season, and the manufacturers produce vaccine based on those recommendations.

The CDC expects a record number of flu vaccine doses to be available this flu season. However, there have been reports that in some cases only partial shipments of vaccine are being delivered. The CDC says it has instructed suppliers to ensure that all orders are at least partially filled early in the season, and that it expects orders to be completely filled by late November.

The CDC encourages anyone who wants a flu shot to get one. Over the past couple of years, temporary shortages of flu vaccine have prompted CDC officials to limit vaccination recommendations to those in high-risk groups. This year the CDC expects there will be enough vaccine to meet demands.

The flu vaccine comes in two forms:

• A shot. A flu shot contains an inactivated vaccine made of killed virus. The shot is usually given in the arm. Because the viruses in the vaccine are killed (inactivated), the shot won't cause you to get the flu, but it will enable your body to develop the antibodies necessary to ward off influenza viruses. You may have a slight reaction to the shot, such as soreness at the injection site, mild muscle ache or fever. Reactions usually last one to two days and are more likely to occur in children who have never been exposed to the flu virus.
• A nasal spray. Administered through your nose, the nasal spray vaccine (FluMist) consists of a low dose of live but weakened flu viruses. The vaccine doesn't cause the flu, but it does prompt an immune response in your nose and upper airways as well as throughout your body.

Plan to get a flu shot in the fall — October or November — so that your body has time to develop antibodies against the virus before the flu season hits its peak.

Both the flu shot and the nasal spray protect you from influenza. But there are differences you should take into consideration before deciding between the two.

Most people who want to reduce the risk of having influenza can get a flu shot. The Centers for Disease Control and Prevention (CDC) recommends the flu vaccine in particular if you:

• Are pregnant
• Are 50 years old or older
• Have a chronic medical condition
• Are a child care worker or health care worker or live with or care for someone at high risk of complications from the flu

Don't get a flu shot if you:

• Have had an allergic reaction to the vaccine in the past.
• Are allergic to eggs.
• Developed Guillain-Barre syndrome, a serious autoimmune disease affecting the nerves outside the brain and spinal cord, within six weeks of receiving the vaccine in the past. People who have experienced Guillain-Barre after the flu vaccine are at higher risk than are others of developing it again.
• Have a fever. Wait until your symptoms improve before getting vaccinated.

Children younger than 9 years old require two doses of the flu vaccine if it's the first time they've been vaccinated for influenza. That's because children don't develop an adequate antibody level the first time they get the vaccine. Antibodies help fight the virus if it enters your child's system. If a flu vaccine shortage were to occur and your child couldn't get two doses of vaccine, one dose might still offer some protection.

If you're over age 65, the vaccine doesn't offer as much protection as it would to someone younger because older adults produce fewer antibodies in response to the virus. Still, the vaccine offers more protection than does skipping the shot altogether. More important, the flu vaccine will decrease the risk of flu-related complications — especially pneumonia, heart attack, stroke and death — to which older adults are especially vulnerable.

An annual flu shot won't protect you specifically from bird flu, but it will reduce the risk of simultaneous infection with human and bird flu viruses. This is important because simultaneous infections are the main way that viruses swap genes and create new strains that potentially can cause flu pandemics.

With or without a flu shot, you can take steps to help protect yourself from the flu and other viruses.

Practice good hygiene
Good hygiene remains your primary defense against contagious illnesses.

• Wash your hands thoroughly and often with soap and water or an alcohol-based sanitizer.
• Avoid touching your eyes, nose or mouth whenever possible.
• Avoid crowds when the flu is most prevalent in your area.
• Cover your mouth and nose with a tissue when you cough or sneeze.

Once you've had the flu, you develop antibodies to the virus that caused it, but those antibodies won't protect you from new or mutated strains of influenza or other viruses.

First of all, don't go to work or school — you'll risk spreading this contagious disease to others.

To relieve your symptoms:

• Drink plenty of fluids.
• Avoid alcohol and tobacco.
• Consider over-the-counter medicine such as acetaminophen (Tylenol, others) to ease the discomfort associated with muscle aches or fever, but don't give aspirin to children or teenagers because of the risk of the rare but serious disease, Reye's syndrome. And keep in mind that acetaminophen can cause severe liver damage if taken in excess.
• Use antiviral medications if prescribed by your doctor, but no longer than recommended. Be sure to monitor yourself. If you start feeling worse, consult your doctor to make sure you're not developing a flu-related complication.

Complications include bacterial infection, pneumonia and dehydration. If you have a chronic medical condition, you may experience a worsening of that condition. Children can develop sinus and ear infections.

Antivirals work both to prevent the flu and to help reduce the severity and duration of the illness by a day or two. Antiviral medications must be taken within 48 hours of the onset of your symptoms and are available only by prescription.

These two antivirals are recommended for treating the flu virus:

• Zanamivir (Relenza)
• Oseltamivir (Tamiflu)

Each of these drugs can cause side effects, including lightheadedness, nausea, loss of appetite and difficulty breathing. They can also lead to the development of antiviral-resistant viruses. Discuss possible side effects with your doctor before starting any antiviral medication.

Your best bet for treating flu symptoms remains the tried and true:

• Rest.
• Get plenty of fluids.
• Take acetaminophen or ibuprofen (Advil, Motrin, others) for aches and pains.

It's also important to eat healthy and get some daily activity to help keep your immune system in top form.

A flu shot is between 70 percent and 90 percent effective in warding off illness due to influenza, depending on the length and intensity of a given flu season and your overall health. A flu shot gives you the best chance for avoiding the flu, but it can't guarantee that you won't get sick. The flu shot protects you against the two or three common influenza viruses. It does not, however, protect you against noninfluenza viral infections, such as those that cause the common cold. Many people mistakenly believe they won't get sick at all if they have a flu shot. Unfortunately, that's not the case. But you are much less likely to catch the most common virus of that flu season.