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Cold, Flu, & Cough Health Center

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Influenza (Seasonal Flu) - Cause

The flu is caused by the influenza virus. Doctors classify the virus as influenza type A and type B, each of which includes several subtypes or strains. Type A is usually responsible for the annual outbreaks that typically occur in the late fall and early winter.

The influenza virus changes often, so having flu caused by one strain does not give you full immunity to other strains.

Recommended Related to Cold & Flu

Myths vs. Facts About Your Immune System

Your immune system is your friend. It protects your body from infection. Give it your full support and, as with any friend, there will be perks. Here’s how it works: Your immune system creates, stores, and distributes the white blood cells that fight bacteria and viruses that enter your body, especially during cold and flu season. For such a simple-sounding process, there's a lot of bad information out there. Here are some myths and facts about the immune system and how it works.

Read the Myths vs. Facts About Your Immune System article > >

  • Widespread outbreaks of the flu usually follow significant changes (called antigenic shifts) in the virus and occur about every 10 years. People who get the flu tend to become much sicker when a shift in the flu virus occurs.
  • Minor changes in the virus (called antigenic drifts) occur nearly every year.

The virus is spread from person to person through:

  • Direct contact, such as shaking hands.
  • Small droplets that form when a person sneezes or coughs.
  • Contact with objects such as handkerchiefs that have been in contact with fluids from an infected person's nose or throat.

When are you contagious?

If you are infected with the flu, you are most likely to pass it to someone else from 1 day before symptoms start and up to 7 days after symptoms develop. Children may be infectious for longer than 7 days after symptoms start.

Symptoms usually develop 1 to 4 days after you are infected. Because symptoms may not develop for a couple of days, you may pass the flu to someone before you know you have it.

WebMD Medical Reference from Healthwise

Last Updated: /2, 14 1
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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