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Nurse giving a vaccine

What is influenza (flu)?

Influenza is a common contagious (easy to catch) viral infection that attacks the respiratory tract (the nose, throat, and lungs).

Most people develop mild-to-moderate symptoms; however, It can cause serious illness and death in some people.

Influenza is also called the flu.

What causes influenza (flu)?

The flu is caused by a virus and there are thousands of different strains of influenza virus. These have been classified into four types: A, B, C, and D.

Only types A and B have been associated with seasonal outbreaks in humans, with A viruses the only ones that have previously caused pandemics. C viruses generally cause mild illnesses and D viruses primarily infect cattle and have not been known to infect humans.

A and B influenza viruses are further classified into either subtypes or lineages. For example:

  • Influenza A: 18 hemagglutinin subtypes (H1 to H18) and 11 neuraminidase subtypes (N1 to N11). Subtypes are named by combining the numbers, for example, A(H1N1), or A(H3N2).
  • Influenza B: Two lineages exist, either: B/Yamagata lineage or B/Victoria lineage. Each is then divided into specific clades and subclades. For example B/Yamagata/16/88 lineage, and B/Victoria/2/87 lineage. Influenza B viruses are slower to mutate or change compared with A viruses.

When naming a new virus, the name starts with the virus type, followed by the place name of where it was isolated, the strain number, the year isolated, and then the subtype. For example, A/Michigan/45/2015 (H1N1) and A/Switzerland/8060/2017 (H3N2).

Occasionally influenza viruses that normally infect animals, such as pigs, start to infect humans. These viruses are called variant viruses and are designated with the letter “v”.

Every year, most influenza cases are caused by three to four major viruses. Officials try and predict what these strains are most likely to be, based on surveillance data, and base each year’s flu vaccine on these predominant strains.

Each year around three to 11% of the U.S. population catches flu, and higher rates of infection and flu severity generally occur when H3N2 is the predominant circulating strain. Flu season in the Northern hemisphere tends to start in October and peaks between December and February, although it can last as late as May. In the Southern hemisphere flu season is typically from May to September.

The flu spreads from person to person when tiny droplets are released from an infected person’s mouth or nose when they cough, talk or sneeze. The flu may also be transferred when people touch a surface or an object that has the flu virus on it and then touch their nose, mouth, or eyes.

What are the symptoms of influenza (flu)?

Symptoms of flu typically come on quickly, but can vary from mild to severe and may include:

  • A fever
  • A cough, which is usually dry
  • A headache
  • A runny and congested nose
  • Fatigue or extreme exhaustion
  • Muscle aches and pains
  • Sore throat.

Children are more likely to have gastrointestinal symptoms, such as vomiting or diarrhea.

People with underlying medical conditions, such as asthma, cancer, diabetes, HIV/AIDS, heart failure, women who are pregnant, and children with neurological problems may experience more severe symptoms or develop complications following the flu, such as pneumonia or inflammation of the heart or brain.

How is influenza (flu) diagnosed?

Not everybody with the flu needs to see a doctor, but you should see a doctor if you have an underlying condition that puts you at high risk of complications or your symptoms are severe. Always ring your doctor's office before you go in to ask for their advice, as you don’t want to infect everybody else who is waiting.

Rapid influenza diagnostic tests are available that can help diagnose the flu and the results are available within 10 to 15 minutes. However, they are not as accurate as other flu tests, so you may still have the flu even though the test reports a negative result.

Most people with flu-like symptoms are not tested because the results do not generally change treatment.

How is influenza (flu) treated?

Treatment depends on the severity of influenza and may include:

  • Antivirals, such as oseltamivir, zanamivir, peramivir or baloxavir marboxil to reduce the severity and shorten the duration of the flu
  • Acetaminophen to relieve pain and headache
  • NSAIDs, such as ibuprofen or naproxen for pain relief
  • Anesthetic lozenges to relieve a sore throat
  • Decongestants or saline nasal drops to unblock a stuffy, congested nose
  • Cough suppressants or honey to help a dry cough.

How is influenza (flu) prevented?

Everybody who is eligible for an influenza vaccination should get one every year. Each year the vaccine is reassessed for appropriateness and changed if appropriate.

Pregnant women are strongly advised to be immunized because pregnancy increases a woman’s chance of developing complications from the influenza virus. Mothers also pass the protection onto their abies who would normally be too young to receive immunization individually.

During flu season everybody should:

  • Wash their hands regularly for 20 seconds and then dry for 20 seconds
  • Avoid sharing drinks, toothbrushes, or utensils with other people
  • Cover their mouth and nose with a tissue when they sneeze
  • Not touch their nose and mouth
  • Avoid crowded places