Influenza - The Flu

About Influenza

Influenza, also known as the flu, is a highly contagious infection of the breathing system (nose, throat and lungs) caused by influenza viruses.

Seasonal influenza is caused by viruses that already circulate among people. Most people have some immunity and a vaccine is available to help prevent the flu. In the United States, flu season usually starts in late fall and lasts throughout the winter, sometimes into early spring.

Avian influenza, also called bird flu, is caused by viruses that occur naturally among wild birds. Sometimes these viruses (such as one called H5N1) can be transmitted from birds to humans. There is no human immunity and no vaccine is available.

Pandemic influenza refers to an outbreak caused by a new flu virus (such as the 2009 H1N1 influenza virus) that spreads around the world. The disease can spread easily from person to person because people have little natural immunity when a new virus is identified. There is no vaccine ready to protect against pandemic influenza because the new virus strain must be identified before a vaccine can be produced, which can take up to six months.  

How Influenza Spreads

The main way influenza viruses spread is from person to person when an infected person coughs, sneezes or talks. When someone with the flu coughs or sneezes, droplets containing the virus are sprayed into the air. Someone else can become infected if they breathe the infected droplets, or touch infected droplets on surfaces and then touch their own eyes, nose or mouth. 

Common Symptoms of the Flu

Symptoms of the flu include:

  • Fever
  • Dry cough
  • Sore throat
  • Runny nose
  • Headache
  • Muscle aches
  • Extreme weakness and tiredness.

Child having temperature takenIn addition, children may have nausea, vomiting and diarrhea.

People develop symptoms one to four days after being infected. They may be contagious (that is, they can spread the virus to others) starting from the day before the first symptom develops until five days after symptoms develop. Children who are infected can spread the virus for more than a week. This is also true of adults who have weakened immune systems.

In general, the seasonal flu is worse than the common cold and symptoms are more intense. People with colds are more likely to have a runny or stuffy nose and typically do not develop serious health problems. Pandemic influenza is much more serious than seasonal influenza and is likely to cause many more deaths. 

Complications Associated with Influenza

In some people, influenza can cause serious complications such as bacterial pneumonia, dehydration and heart failure. The people most at risk of complications for seasonal influenza are:

  • Children younger than 2 years
  • Adults 65 and older
  • People with weakened immune systems and chronic illnesses such as asthma, congestive heart failure and diabetes

High risk groups may vary as more is learned about new influenza viruses and which populations are most affected.

Each year, more than 200,000 people in the United States are hospitalized from complications of the flu, and about 36,000 die as a result.

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