Influenza, or flu, is a respiratory infection caused by a variety of flu viruses. The most familiar aspect of the flu is the way it can "knock you off your feet" as it sweeps through entire communities.

The flu differs in several ways from the common cold, a respiratory infection also caused by viruses. For example, people with colds rarely get fevers or headaches or suffer from the extreme exhaustion that flu viruses cause.

Besides the rapid start of the outbreaks and the large numbers of people affected, the flu is an important disease because it can cause serious complications. Most people who get the flu get better within a week (although they may have a lingering cough and tire easily for a while longer). For elderly people, newborn babies, and people with certain chronic illnesses, however, the flu and its complications can be life-threatening.

You can get the flu if someone around you who has the flu coughs or sneezes. You can get the flu simply by touching a surface like a telephone or door knob that has been contaminated by a touch from someone who has the flu. The viruses can pass through the air and enter your body through your nose or mouth. If you've touched a contaminated surface, they can pass from your hand to your nose or mouth.

Typically, the fever begins to decline on the second or third day of the illness. The flu almost never causes symptoms in the stomach and intestines. The illness that some call "stomach flu" is not influenza.

The main way to keep from getting flu is to get a yearly flu vaccine. You can get the vaccine at your doctor's office or a local clinic, and in many communities at workplaces, supermarkets, and drugstores. You must get the vaccine every year because it changes.

Scientists make a different vaccine every year because the strains of flu viruses change from year to year. Nine to 10 months before the flu season begins, they prepare a new vaccine made from inactivated (killed) flu viruses. Because the viruses are killed, they cannot cause infection. The vaccine preparation is based on the strains of the flu viruses that are in circulation at the time. It includes those A and B viruses (see section below on types of flu viruses) expected to circulate the following winter.

Sometimes, an unpredicted new strain may appear after the vaccine has been made and distributed to doctor's offices and clinics. Because of this, even if you do get the flu vaccine, you still may get infected. If you do get infected, however, the disease usually is milder because the vaccine still will give you some protection.

If you do get the flu and want to take medicine to treat it, your health care provider may prescribe one of four available antiviral medicines.

Tamiflu (oseltamivir) is for adults and children 1 year and older and Relenza (zanamivir) is for adults and children 7 years and older who have an uncomplicated flu infection and who have had symptoms for no more than 2 days. Both treat influenza type A and type B infections.

Flumadine (rimantadine) helps adults who have influenza type A virus infections. It has no effect on influenza type B virus infections.

Symmetrel (amantadine) may be taken by adults and children who are 1 year of age and older to prevent and treat type A influenza virus infections. Symmetrel, however, is more likely to cause side effects such as lightheadedness and inability to sleep more often than is Flumadine.

To work well, you must take these medicines within 48 hours after the flu begins. They reduce the length of time fever and other symptoms last and allow you to return to your daily routine quicker.