Every winter, like clockwork, the flu returns. It infects millions of us -- about 5 to 20 percent of the U.S. population alone, according to the Centers for Disease Control and Prevention (CDC). Health Canada estimates that 10 to 25 percent of Canadians get the flu each year. It leaves us sniffling, sneezing, coughing, achy and generally feeling miserable for anywhere from a few days to a few weeks.

What is the Flu?
The flu is a respiratory illness caused by the influenza virus. The flu is not the same as a cold, although they share many of the same symptoms. The cold is caused by a different virus, and it tends to have milder symptoms than the flu. Colds are also less likely to cause serious complications.

Flu symptoms can include any or all of the following:

Body aches
Runny nose and/or congestion

These symptoms, although uncomfortable, are generally not dangerous. But the flu also weakens the immune system, leaving it vulnerable to more serious infections. High-risk individuals (see Who is at Risk?) in particular are susceptible to serious complications, such as:

Bacterial pneumonia
Sinus problems and ear infections (primarily in children)
Worsening of preexisting conditions, such as asthma or diabetes

How do People Get the Flu?

Flu season in North America runs from November through March, but dates can vary from year to year. January and February tend to be the most active flu months.

The "Stomach Flu"
The term "stomach flu" is actually a misnomer. Vomiting, diarrhea and stomach aches can be caused by a virus, but they are rarely related to the flu. The flu is a respiratory illness, not a gastrointestinal one.

How is the flu spread?
The flu is highly contagious. It is spread primarily by coughing and sneezing (which people who have the flu tend to do a lot of). Let's say you have the flu virus. Every time you cough or sneeze, you release tiny droplets of fluid into the air. Those tiny droplets can fly pretty far -- up to 3 feet (about 1 meter). If some of those droplets land on the nose or mouth of a person standing nearby, that person is likely to get as sick as you are, usually within one to four days. You can also spread the virus if you touch something (like a doorknob or table) after you've sneezed or coughed into your hand, and then other people come along and touch the same doorknob or table and put their hand on their nose or mouth.

If you have the flu, you're not just contagious when you have symptoms. You can pass along the virus one day before you start sniffling and sneezing, and you can keep passing it along for seven days after you start sniffling and sneezing. Children can be contagious even beyond the seven days.

Who is at Risk?
Anyone can get the flu, but some groups are more susceptible than others and are at greater risk for more serious complications or even death.
Risk groups include:

Children under the age of 2 (whose immune system is not yet fully developed)
Seniors over the age of 65 (most flu deaths are among seniors)
Anyone who has a chronic medical condition (such as asthma or diabetes)
Pregnant women
Health care workers
Nursing home residents

How Can You Treat the Flu?

Unfortunately, there isn't a pill or a liquid you can take that will "cure" you of the flu. Penicillin and other antibiotics won't work, because they only kill bacteria, and the flu is caused by a virus.
There are, however, a few approved antiviral drugs, including Symmetrel®, Flumadine®, Relenza® and Tamiflu®, that have been shown to shorten the duration of the illness.

Relenza® (zanamivir) and Tamiflu® (oseltamivir phosphate) are neuraminidase inhibitors. They work by blocking the action of a protein called neuraminidase, which sits on the surface of a cell and normally helps the influenza virus enter and leave the cell. Neuraminidase inhibitors trap the virus once it enters a cell.

By stopping the virus from spreading to other cells, Relenza® and Tamiflu® lessen the severity and shorten the duration of the flu infection.

Symmetrel® and Flumadine® also lessen the severity and shorten the duration of the flu, but they only work against influenza A. Both are antiviral medications that work by stopping the virus from replicating.

All four drugs are by prescription only and do have potential side effects, so they should only be taken with the advice of a doctor.

The best advice for treating the flu is to rest and drink plenty of liquids. Over-the-counter cold and flu remedies can alleviate some of the symptoms, at least temporarily. Aspirin may relieve fever and aches, but it should not be given to children and adolescents because of the risk of a rare but potentially dangerous illness called Reye's Syndrome.

How Can You Avoid Getting the Flu?

Experts say the best way to avoid catching the flu is to practice good hygiene during flu season. Here are a couple of tips:
Wash your hands throughout the day with warm water and soap.
Avoid anyone who is coughing or sneezing.
If you do get sick, you can avoid infecting others if you:

Stay home until you're feeling better.
Cover your mouth and nose with a tissue whenever you cough or sneeze.
If you have to sneeze or cough into your hands, wash them thoroughly afterward with warm water and soap.
Remember that you can spread the flu for up to seven days after you get sick, so be careful with your germs even after most of your symptoms have passed.
In the next section, we'll take a look at the flu vaccine and see how it wards off this illness.

The Flu Vaccine
Another way to prevent the flu is by getting a vaccine at the beginning of each flu season (October or November). The earlier you get vaccinated the better, because it takes about two weeks for the vaccine to take its full protective effect. Children under the age of 9 who have never had a flu shot especially need to get an early start, because they will need to have two vaccinations administered about one month apart.
The flu vaccine works by triggering your body's immune system response. When you get a flu vaccine, your body recognizes the flu virus as a foreign invader and produces antibodies to it. The next time your body encounters the flu virus, it will remember that it is a hostile invader and quickly launch an immune attack to kill off the virus.

But if your body remembers the virus, why do you need to get a flu shot every year? First, because flu strains differ from year to year; and second, because immunity declines over time.

The flu vaccine comes in two forms: a shot and a nasal spray.