Influenza is an infection caused by a virus. It can affect anyone of any age. It spreads rapidly from person to person when people are in large groups, such as at school, at work, in daycare centers or at public places or events. Flu usually hits between October and May, peaking in late December through early March.

The flu virus attacks a person's nose, throat and lungs. Although people sometimes speak of "stomach flu" when referring to diarrhea, nausea and vomiting caused by a virus, this is not true influenza.

Symptoms of flu can range from being as mild as the common cold to being fatal. During the 1918 influenza epidemic, at least 20 million people worldwide died. Each year, more than 35,000 Americans die from influenza and another 150,000 require hospitalization.


Colds and influenza have some common symptoms, such as coughing and sneezing. Individuals with the flu, however, usually feel worse, take longer to fully recover and may experience other symptoms, such as:

  • Chills and sweats
  • Fatigue
  • Fever of 101° to 106° F
  • Fever that lasts from one day up to one week
  • Loss of appetite
  • Muscle aches (especially in the back, arms and legs)
  • Sore throat
  • Stuffy nose
  • Weakness

Symptoms of the virus usually appear one to four days after exposure to the virus. Individuals generally feel better in one week to 10 days. Flu can lead to serious lung infections, such as pneumonia or swelling of the lining of the airways in the lungs (bronchitis).

Causes and Risk Factors

Influenza is caused by three types of viruses:

Influenza A, which was responsible for the deadly worldwide epidemic in 1918. These serious epidemics usually strike every 10 to 40 years.

Influenza B, which causes smaller epidemics that are limited to specific localities. These types of epidemics usually appear every three to 15 years. Because global travel is so much more accessible today, this type of influenza can quickly spread to other places.

Influenza C, which is rare and causes mild symptoms. It is fairly stable and unchanging.

Types A and B are continually changing, making it difficult to create a vaccine that protects against the flu from one year to the next.

After having a strain of influenza virus, a person's body develops antibodies to the virus that protect from getting that particular strain again. However, because the strains change so often, people tend to get the flu more than once.

People who are at greatest risk of contracting influenza are those who:

  • Are between the ages of six months and two years
  • Are children with asthma or other chronic lung conditions
  • Are in the fourth to ninth month of pregnancy
  • Are older than 50
  • Have a chronic disease, such as diabetes, heart disease or kidney or lung conditions (People with diabetes are more likely to have complications from influenza, such as pneumonia, and are three times more likely to die of flu than are other people.)
  • Have a weakened immune system from HIV or AIDS, leukemia, chemotherapy or from taking anti-rejection medications after an organ transplant
  • Have sickle-cell anemia
  • Live in a nursing home or long-term care facility

Individuals who suspect that they have the flu should see their doctor as soon as possible, especially if at high risk. People with serious symptoms, such as difficulty breathing, severe sore throat, coughing that produces a lot of green or yellow fluid or feelings of being faint, should see a doctor quickly because these may be signs of bacterial pneumonia. Other serious signs include severe cough, high fever and sharp pain when breathing deeply.

Having a flu shot each year may help lower your risk of catching influenza. The vaccine can be given to anyone who is six months or older. Children who are not at risk but who spend a great deal of time with someone who is, such as a grandparent, may need to be vaccinated to prevent the spread of flu.


To diagnose flu, a doctor will take a medical history and perform a physical examination. Flu can normally be diagnosed on the basis of the symptoms, the season of the year and local public health conditions.


A doctor may recommend bed rest and drinking plenty of fluids. Within the first day or two of having flu symptoms, certain drugs that fight viruses may be helpful.

Neuraminidase inhibitors are used to treat both influenza A and B. These antiviral drugs make inactive an enzyme that the virus needs to grow and spread. Zanamivir (Relenza®) is a neuraminidase inhibitor that is inhaled every 12 hours for five days, and oseltamivir (Tamiflu®) is a neuraminidase inhibitor in pill form. These drugs can shorten the length of the flu by a day or two and may help prevent spreading it to other family members.

Antiviral pills, such as amantadine (Symmetrel®) and rimantadine (Flumadine®) may help alleviate influenza A, if started soon after symptoms begin. They do not affect the course of influenza B, and they may produce side effects, such as nausea, nightmares and convulsions.

These steps may help flu sufferers feel better:

  • Get plenty of sleep
  • Drink plenty of liquids
  • Have some chicken soup, which can help break up congestion as well as provide nutrition and fluid
  • Take an over-the-counter pain reliever, such as acetaminophen, aspirin or ibuprofen. These will not decrease the flu's duration, but they can help those infected feel better. Be aware of the side effects, which can include stomach pain, bleeding and ulcers. Acetaminophen may harm the liver if taken for a long period of time or in large doses. Do not give acetaminophen to children without a doctor's recommendation because it may cause liver or kidney damage. Aspirin should not be given to anyone younger than 16 because it may cause Reye's syndrome, a rare but potentially fatal disease.

The following steps are the best protection against getting influenza:

  • Eat a balanced diet, and exercise regularly
  • Get enough sleep, and do not get run down
  • Wash hands often
  • Get a flu shot every fall between mid-October and mid-November so that your ability to resist infection by the virus is at its highest during the peak period of the flu season from December to March (Remember that the effectiveness of flu shots varies from year to year as the virus changes. The strains used in the vaccine are selected nine months before each flu season. The actual strain that dominates a given season may be different than the one used for the vaccine.)
  • Avoid crowds during the peak period of the flu season if at risk of getting the flu

Adults receive the flu vaccine in a single injection. Children younger than nine who are getting their first flu shot will need two shots about a month apart. The vaccine requires one to two weeks to take effect.

The vaccine is made from a form of the virus that doesn't have the power to infect you with the disease. No one can get the flu from a flu shot. Individuals may be sore where the shot was given or have mildly achy muscles or a fever that starts six to 24 hours after getting the shot. These symptoms last only a day or two and are most likely to happen in children who have never been exposed to the flu virus.

People who should not have a flu shot are individuals who are:

  • Allergic to eggs because the viruses are grown in chicken eggs and may cause an allergic reaction
  • In the first three months of pregnancy, unless a doctor directs otherwise