Photo courtesy CDC Influenza virus

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.

Although most of us think of the flu as a mild annoyance that we have to deal with each winter, it can actually be a very dangerous disease. The CDC estimates that in the United States alone, more than 200,000 people are hospitalized with the flu or with flu-related complications each year, and more than 30,000 people die from it. Around the world, the flu kills close to a half million people every year.

In this article, we'll find out how people get the flu, what the symptoms are and how to protect yourself.

2004 Vaccine Shortage In 2004, the British company Chiron announced that none of its flu vaccine (Fluvirin®), about 46 million doses, would be available for the 2004-05 flu season. Without Chiron's vaccines, the nation's flu vaccine supply was cut in half. By early October 2004, 30 million doses of the 54 million still available flu vaccines had already been given out. Lines for flu shots snaked around the block outside of supermarkets and health-department offices. To conserve the remaining vaccines for those who needed it most, the CDC asked that only people in high-risk groups get vaccinated. The rest of the world did not report any shortages, primarily because many countries in Asia and Europe produce their own vaccine supplies and because other countries have different policies governing the administration of flu shots. Read Press Release: Chiron will not supply Fluvirin® influenza virus vaccine for the 2004-2005 influenza season to learn more.

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.

When the influenza virus gets into the body, it moves into the respiratory tract. Once there, it binds to the surface of cells. The virus then opens and releases its genetic information (RNA) into the cell's nucleus. The nucleus is where the cell's genetic information (DNA and RNA) is stored. The virus replicates, or copies itself, and takes over the functions of the cell. The copies of the virus move to the cell membrane until the cell finally dies and releases them out into the body, where they go on to infect other cells.

The respiratory tissues swell up and become inflamed (the inflammation usually heals within a few weeks). As the virus moves through the respiratory tract and into the bloodstream, the first symptoms begin to emerge. The replication process continues for up to several days, until the body's immune system begins to fight the virus off.

Flu symptoms can include any or all of the following:

• Coughing
• Sneezing
• Fever
• Body aches
• Runny nose and/or congestion
• Tiredness

Flu Strains You may have heard the flu referred to as "strain A" or "strain B." After the flu virus was first identified in the 1930s, scientists classified it into three strains: A, B and C. Type A is the most common and most severe form of the flu. Type B is milder and less prevalent. Type C viruses typically don't cause large-scale epidemics.

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

How do People Get the Flu?

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.

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.

During flu season, people begin coming down with the illness, and they quickly spread it to friends, family and coworkers. Schools are particularly notorious for spreading the flu, because students are in such close quarters. And when a child picks up the virus, he or she often brings it home and shares it with the rest of the family. The National Institute of Allergy and Infectious Diseases estimates that one out of every three families of school-aged children is infected with the flu each year.

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.

The 1918 Flu Pandemic Normally, the flu only spreads to people in the country in which it originated. But sometimes, the illness can travel and infect people around the world. This rampant spread is called a pandemic. The worst flu pandemic in history struck in 1918. Between 1918 and 1919, the pandemic killed more than 500,000 people in the United States and more than 20 million people worldwide. More lives were lost in this flu pandemic than in all of the 20th century wars combined. Photo courtesy U.S. Navy Poster issued by the U.S. Treasury Department during the 1918 flu pandemic

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

What is the Avian Flu? You may have heard talk on the news about the avian flu. The avian flu is a type of the A strain virus that infects birds. Typically, humans cannot catch the flu from birds, but a few bird-to-human outbreaks have been reported since the late 1990s. Most of them have been in Asia. People were infected when they came into contact with sick birds or with contaminated surfaces. Most had flu-like symptoms, but some had more serious complications, including pneumonia and acute respiratory distress.

How Can You Treat the Flu?

Photo courtesy CSIRO In this electron micrograph, you can see neuraminidase "spikes" surrounding the influenza virus.

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.

Photo courtesy CSIRO Health Sciences and Nutrition Relenza® (zanamivir) releases beads that bind to neuraminidase proteins, stopping the virus from leaving the infected cell and spreading to others.

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

Photo courtesy CSIRO Health Sciences and Nutrition Left: Relenza® powder-filled capsules and inhaler device; Right: Relenza® beads in an active neuraminidase pocket

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?

Photo courtesy Simcoe County District Health Unit

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.

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.

Photo courtesy U.S. Army U.S. Army soldier receiving the flu vaccine in shot form

The shot, which is normally given in the arm, is made up of three different viruses. The three strains are chosen by scientists working in laboratories around the world. They collect flu viruses and predict which strains will be most prevalent in the coming flu season. The viruses in the shot are inactivated, or dead, which means that they can't actually give you the flu.

The nasal-spray flu vaccine is often referred to as LAIV (Live Attenuated Influenza Vaccine). Unlike the flu shot, it uses a live but weakened version of the virus. Like the shot, it contains three different flu strains. When the LAIV is sprayed into the nose, it works much like the injected form of the vaccine, stimulating the immune system to develop antibodies against the virus.

How effective is the nasal vaccine? One large study found that it reduced the incidence of flu in young children (age 1 to 7) by 92 percent. The study didn't test the effectiveness of the flu spray on adults. Because it is a live vaccine, LAIV is only recommended for healthy people between the ages of 5 and 49.

Who should be vaccinated?

Photo courtesy Simcoe County District Health Unit Flu vaccine vial

Anyone in a high-risk group should be vaccinated at the beginning of every flu season. The CDC recommends that all children, ages 6 to 23 months, get vaccinated. Very young children are more likely to be hospitalized with the flu, and to die from it, than older children and adults. Because infants under 6 months are too young to take the vaccine safely, all people around them (family members and child care workers) should be vaccinated, as well.

Older adults (over age 65) should also be vaccinated, as well as anyone with a chronic health condition like asthma or diabetes. Plus, the CDC recommends that pregnant women and people who work in the healthcare industry be vaccinated.

Who should not be vaccinated?
People who are allergic to chicken eggs should not be vaccinated, because the flu vaccine is grown in eggs and could cause a severe allergic reaction.

Other people who shouldn't receive the flu shot are:

• People who have had a severe reaction to the flu shot in the past
• Anyone who developed Guillain-Barr