What Is Influenza?

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The flu is a highly contagious and common illness that is caused by the influenza virus. There are two main types of flu viruses: types A and B, both of which cause illness in humans.

People can get the flu any time of year, but in the United States and most of the Northern Hemisphere, flu season spans late fall to early spring. Flu activity typically peaks between December and March.

People of all ages can get the flu. However, children, the elderly, and people with weakened immune systems or chronic medical conditions are most susceptible and more likely to develop serious complications.

Influenza Types
 Illustration by Joshua Seong. © Verywell, 2017.

Causes of the Flu

There are many different strains of the flu virus, and they mutate often. This is why people continue to come down with the flu year after year. The flu is a very contagious illness that spreads easily. Coughs and sneezes are strong enough to transmit droplets from the mouth and nose. You can also get the flu through personal contact (handshakes or hugs), saliva (kissing or sharing drinks), and by touching contaminated surfaces (doorknobs or faucets).

When someone else breathes in those respiratory droplets or touches any contaminated object and then touches their nose, mouth, or eyes, the virus spreads.

A person is contagious with the flu from one day before symptoms appear to up to five days after becoming sick. It's possible to spread the flu before you even know you have it.

What to Expect

The flu typically lasts between four and five days, although symptoms can last anywhere from two to seven days.

 Some flu symptoms may be similar to cold symptoms, including:

  • Muscle or body aches
  • A cough
  • Runny or stuffy nose
  • Itchy or watery eyes
  • A sore throat
  • Fatigue
  • Low fever

The key difference between cold and flu symptoms, however, is severity. A cold tends to start slowly and gradually get worse over a few days. Symptoms can make you feel pretty lousy, but they usually aren't severe enough to disrupt your life. The flu hits you all at once and completely wipes you out, rendering you unable to go about your daily routine.

Common symptoms of the flu include:

If you think you might have the flu and you or someone you will be exposed to is at high risk for complications, contact your health care provider as soon as you develop symptoms. He or she can perform a flu test or diagnose you based on your symptoms and determine the best course of treatment for you.

Influenza A

Influenza is a virus that actually has hundreds of different strains. The virus mutates frequently, but the strains are classified into one of three categories—A, B, or C. Influenza A is the group that most commonly causes illness in humans.

All influenza A viruses are further broken down into H and N subtypes. So, any influenza virus that is described as "H#N#" (such as H1N1) is an influenza A virus. There are 18 H subtypes and 11 N subtypes, but only three combinations have actually caused highly contagious illness in humans. Other combinations have been found to infect other species (such as birds and pigs), but they have not caused widespread human infections. The three combinations that cause almost all outbreaks of the flu in humans are H1N1, H2N2 and H3N2.

Even in these subtypes, the influenza virus can mutate and change each year. For this reason, influenza viruses are also named using:

  • The host of origin (swine, chicken, etc., or no host if it is of human origin)
  • The geographical location of origin (Hong Kong, Alberta, etc.)
  • Strain number
  • Year of discovery (or isolation)

When the U.S. Centers for Disease Control and Prevention and the World Health Organization name a new strain of the influenza virus, they start with the group (A, B, or C), then list the host, location of origin, strain number, year of discovery, and H-N subtype in parentheses.

An example of an influenza A virus looks like this:

A/duck/Alberta/35/76 (H1N1)
This indicates a virus of duck origin, identified in Alberta, Canada, strain 35, in 1976.

History of Influenza A

All of the major flu pandemics in modern history have been caused by influenza A viruses. The 1918 pandemic—also known as the Spanish flu—was caused by an H1N1 virus. The 1957 flu pandemic—also called the Asian flu—was caused by an H2N2 virus. The 1968 pandemic—also called the Hong Kong flu—was caused by an H3N2 virus. Finally, the 2009 pandemic—called swine flu—was caused by a novel H1N1 virus.

Influenza A in Flu Vaccines

The seasonal flu vaccine typically contains two different strains of influenza A and one or two strains of influenza B. The strains included in the vaccine are the same for all types of flu vaccines in a given year but may change from year to year.

Influenza B

Influenza B is less common but still causes outbreaks of seasonal flu. One or two strains of influenza B are included in the seasonal flu vaccine every year to protect people from the strain(s) that researchers believe are most likely to cause illness during the upcoming flu season. The quadrivalent (four-strain) flu vaccine contains two strains of influenza B, while the traditional trivalent (three-strain) flu vaccine only contains one. 

Influenza B is not broken down into subtypes like influenza A, but it is broken down into individual strains.

What Influenza B Means for You

There is not much difference between influenza A and B when it comes to how they affect you; one is not more or less severe than the other. The major difference comes down to how they are classified and their potential to cause epidemics. Influenza B can cause outbreaks of seasonal flu, but they occur less frequently than outbreaks of influenza A.

Is There a Cure for the Flu?

There is no cure for the flu. There are some prescription antiviral medications, such as Tamiflu, that may help shorten the duration of the illness. However, Tamiflu is only effective if taken within the first 48 hours of the onset of symptoms. Your doctor will determine if these medications are right for you.

Cold and Flu Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Man

What About the Flu Shot?

The flu shot is typically available in the United States starting in August or September. It provides protection against the flu strains researchers believe are most likely to cause illness during the upcoming flu season. While it isn't a guarantee because the influenza virus mutates so frequently, it's still our best bet for protection from the flu.

Typically, two strains of influenza A and one strain of influenza B are included in the seasonal flu vaccine. Quadrivalent flu vaccines contain two strains of influenza A and two strains of influenza B.

A Word From Verywell

If you come down with the flu, take care of yourself, try to get plenty of rest, stay hydrated, and stay away from other people so you don't spread the virus. If you are at high risk for complications from the flu or you believe you may have developed a secondary infection, be sure to see your health care provider as soon as possible.

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