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.
The Centers for Disease Control and Prevention (CDC) estimates that 10 to 20 percent of Americans come down with the flu during each flu season, which typically lasts from November to March. Children are two to three times more likely than adults to get sick with the flu, and children frequently spread the virus to others. Although most people recover from the illness, CDC estimates that in the United States more than 100,000 people are hospitalized and about 36,000 people die from the flu and its complications every year.
Flu outbreaks usually begin suddenly and occur mainly in the late fall and winter. The disease spreads through communities creating an epidemic. During the epidemic, the number of cases peaks in about 3 weeks and subsides after another 3 or 4 weeks. Half of the population of a community may be affected. Because schools are an excellent place for flu viruses to attack and spread, families with school-age children have more infections than other families, with an average of one-third of the family members infected each year.
IMPORTANCE OF FLU
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.
You are at greatest risk of getting infected in highly populated areas, such as in crowded living conditions and in schools.
If you get infected by the flu virus, you will usually feel symptoms 1 to 4 days later. You can spread the flu to others before your symptoms start and for another 3 to 4 days after your symptoms appear. The symptoms start very quickly and may include
- Body aches
- Dry cough
- Sore throat
- Stuffy nose
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.
Usually, health care providers diagnose the flu on the basis of whether it is epidemic in the community and whether the person’s complaints fit the current pattern of symptoms. Health care providers rarely use laboratory tests to identify the virus during an epidemic. Health officials, however, monitor certain U.S. health clinics and do laboratory tests to determine which type of flu virus is responsible for the epidemic.
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.
Until recently, you could get the flu vaccine only as an injection (shot). In 2003, however, the Food and Drug Administration (FDA) approved a nasal spray flu vaccine called FluMist, which you can get from your health care provider. FDA approved it for use in healthy people aged 5 to 49 years.
You should not use FluMist if
- You have certain lung conditions, including asthma, or heart conditions
- You have metabolic disorders such as diabetes or kidney dysfunction
- You have an immunodeficiency disease or are on immunosuppressive treatment
- You have had Guillain-Barré syndrome
- You are pregnant
- You have a history of allergy or hypersensitivity, including anaphylaxis, to any of the parts of FluMist or to eggs
Children or teenagers who regularly take aspirin or products containing aspirin also should not take FluMist.
Your immune system takes time to respond to the flu vaccine. Therefore, you should get vaccinated 6 to 8 weeks before flu season begins in November to prevent getting infected or reduce the severity of flu if you do get it. Because the flu season usually lasts until March, however, it’s not too late to get it after the season has begun. The vaccine itself cannot cause the flu, but you could become exposed to the virus by someone else and get infected soon after you are vaccinated.
Possible side effects
You should be aware that the flu vaccine can cause side effects. The most common side effect in children and adults is soreness at the site of the vaccination. Other side effects, especially in children who previously have not been exposed to the flu virus, include fever, tiredness, and sore muscles. These side effects may begin 6 to 12 hours after vaccination and may last for up to 2 days.
Viruses for producing the vaccine are grown in chicken eggs and then killed with a chemical so that they can no longer cause an infection. The flu vaccine may contain some egg protein, which can cause an allergic reaction. Therefore, if you are allergic to eggs or have ever had a serious allergic reaction to the flu vaccine, CDC recommends that you consult with your health care provider before getting vaccinated.
If you are in any of the following groups or live in a household with someone who is, CDC recommends that you get the flu vaccine.
- You are 50 years of age or older
- You have chronic diseases of your heart, lungs, or kidneys
- You have diabetes
- Your immune system does not function properly
- You have a severe form of anemia
- You will be more than 3 months pregnant during the flu season
- You live in a nursing home or other chronic-care housing facility
- You are in close contact with children 0 to 23 months of