Influenza, commonly called the flu, is a respiratory illness caused by a specific kind of virus. Compared to most viruses that cause upper respiratory illnesses (e.g., the common cold), influenza viruses usually cause a more severe illness that is more likely to lead to serious medical complications, such as pneumonia.

What kinds of viruses cause influenza?

Influenza viruses are divided into three types: A, B and C. Types A and B are responsible for the outbreaks of respiratory illness that occur almost every year and often are associated with increased rates of hospitalization and death. Currently there are three different influenza strains in worldwide circulation: two type A viruses and one type B. (Type C differs because it usually causes either a very mild respiratory illness or no symptoms at all.) However, influenza viruses continually change over time (usually by mutation). This enables a virus to evade a person's immune system, making people susceptible to influenza infection throughout their lives.

Occasionally, type A viruses can change abruptly and a new subtype will suddenly emerge. When this occurs, large numbers of people--sometimes an entire population--have no antibody protection. This results in a worldwide epidemic, or pandemic. There have been three influenza pandemics this century. In 1918-19, the Spanish flu caused approximately 500,000 deaths in the United States and 20 million worldwide. The Asian flu resulted in 70,000 U.S. deaths in 1957-58, and the Hong Kong flu of 1968-69 claimed 34,000 lives in the United States. However, these are rare events.

When is influenza most common?

The influenza season, when the illness is most common, usually occurs during the colder months of the year: late fall, winter and early spring.

What are the symptoms of influenza?

Typical symptoms include fever--often with chills and headache, muscle aches and fatigue. Respiratory symptoms are a cough, sore throat, and a runny or stuffy nose. Although nausea, vomiting and diarrhea can sometimes accompany an influenza infection, especially in children, gastrointestinal symptoms are rarely prominent. The term "stomach flu" is incorrectly used sometimes to describe gastrointestinal illnesses caused by other microorganisms.

How many people are affected by influenza each season?

During most years, between 10 percent and 20 percent of the population are infected with influenza viruses. Children get influenza more often than healthy adults. Older adults and people of any age with chronic illnesses are more apt to suffer from serious medical complications from influenza. In an average year, approximately 36,000 Americans die from influenza and its complications and many more are hospitalized.

What can be done to prevent influenza?

The best prevention is vaccination against flu. Because influenza viruses change frequently and because protection from the vaccine decreases with time, people should get vaccinated every year. The amount of vaccine manufactured in the United States has increased substantially in recent years. During the 2003-2004 influenza season, there were 88 million doses of vaccine available. While most people get shots against influenza, there is a nasal spray vaccine product for healthy persons between the ages for five and 49.

There are several antiviral drugs available that are effective in preventing and treating influenza. These drugs are available by prescription and are prescribed specifically for type A or B infections. However, they must be taken before or within 48 hours of onset of illness.

Who should get an influenza shot?

Anyone who wants to reduce his or her risk of getting influenza should get an annual flu vaccination. Vaccination is particularly important, though, for certain people at risk of complications from influenza:

• persons 65 years of age and older, even if they are otherwise in good health;
• residents of nursing homes and long-term care facilities;
• persons of all ages who have chronic heart or lung conditions, including asthma;
• persons with diabetes, renal disease, severe anemia or suppressed immune systems;
• children who are on aspirin therapy; and
• health care providers or household members of any of the above groups.

Are there any good reasons not to get a flu shot?

Persons who have a severe allergy to eggs or who have had a previous allergic reaction to influenza vaccine should not get a flu shot without consulting a physician.

Many people avoid getting vaccinated for the wrong reasons:

• The vaccine causes unpleasant side effects or may even cause the flu. Influenza vaccine causes no side effects in most people. Less than one-third of those who get flu shots have some soreness at the vaccination site and about 5 percent to 10 percent experience mild side effects such as headache or low-grade fever for about a day after getting the shot.
• Flu shots are not very effective. Overall vaccine effectiveness varies from year to year, depending on how similar the influenza virus strains included in the vaccine are to those that are circulating during the flu season. Because vaccine strains are chosen nine to 10 months before the flu season and because influenza viruses mutate over time, the circulating strains can change between the time the vaccine strains are chosen and the next flu season. These changes in the virus(es) sometimes reduce the ability of the vaccine-induced antibodies to inhibit the newly mutated virus, thereby reducing the vaccine's effectiveness. Vaccine effectiveness also varies from one person to another. Studies of healthy young adults have shown influenza vaccine to be 70 percent to 90 percent effective in preventing illness. In the elderly and those with chronic medical conditions, the vaccine is often less effective in preventing influenza than in reducing the severity of the illness and the risk of serious complications and death. Research has shown the vaccine to reduce hospitalization by about 70 percent and death by about 85 percent in independent seniors. Among nursing home residents, vaccine can reduce the risk of hospitalization by about 50 percent, the risk of pneumonia by about 60 percent and the risk of death by 75 percent to 80 percent.
• Flu shots are expensive for those on fixed incomes. Since May 1993 Medicare Part B has paid for flu shots for the elderly, the country's largest high-risk group. Almost half of eligible Medicare part B persons in Illinois still do not receive vaccine.

When is the best time to get a flu shot?

In the United States, influenza activity is typically very low until December; peak activity most often occurs between January and March. Influenza vaccine should be administered between September and mid-November. It takes about one to two weeks after vaccination for antibodies against influenza to develop and provide protection.

What else can be done to avoid getting the flu?

• Wash hands frequently with soap and warm water.
• Cover your nose and mouth when you cough and sneeze, preferably with a facial tissue or your arm, not your hands.
• Avoid close contact with people who are sick.
• When you are sick, keep your distance from others to protect them from getting sick. Stay home from work or school until you recover.
• Contact your medical provider if you are experiencing severe symptoms that you believe require medical attention.
• Do not share things that go into the mouth, such as drinking cups, straws, etc.
• Frequently clean commonly touched surfaces if someone in your house has a cold or the flu.
• Avoid touching your eyes, nose or mouth while in public as these areas act as portals for bacteria and viruses to enter your body.