Influenza Virus: Causes, Symptoms & Treatment

Influenza Virus: Causes, Symptoms & Treatment
This digitally-colorized negative-stained transmission electron micograph (TEM) shows a number of influenza A viruses. H1N1 is a strain of influenza A. Credit: CDC/ F.A. Murphy

Influenza Virus: Causes, Symptoms & Treatment

Influenza Virus: Causes, Symptoms & Treatment.

The flu (short for influenza) is a respiratory virus that affects the ( throat, nose, bronchi and, sometimes, the lungs.) There are different types of influenza viruses and they evolve and change from year to year. Many people use the word “flu” when they actually have a cold. Although the common cold is also caused by viruses, the flu and common cold differ in several ways.

In North America, the flu almost always strikes between November and April. Up to 25% of the population may be infected in an average year. Stronger epidemics (i.e., when the flu occurs in more people than expected in a given area or season) come every 2 or 3 years, infecting twice as many people as during an “off” year.

History Of Influenza Virus

The word “influenza” in Italian literally means “influence,” a word that Italians have used for illnesses since at least the 1500s because they, like others at the time, believed that the stars influenced health, according to the Douglas Harper Etymology Dictionary. There have been many major pandemics caused by the flu throughout history.

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For example, the 1918 to 1919 pandemic, known as “The Great Pandemic,” infected 20 to 40 percent of the worldwide population and an estimated 50 million people died because of it, according to the Centers for Disease Control and Prevention (CDC). This pandemic was also named “Spanish flu” because is believed that the pandemic originated in Spain.

A more recent pandemic occurred in 2009 to 2010, when a new form of the influenza strain H1N1 appeared. This virus is also called “swine flu” because the virus is similar to a virus found in pigs (not because it can be contracted from pigs or by eating pork).

For most people, the flu is an inconvenience that subsides in a few days. Globally, 5 to 10 percent of adults and 20 to 30 percent of children get the flu each year and 3 to 5 million of these cases are severe, leading to about 250,000 to 500,000 deaths, according to the World Health Organization (WHO).

In the United States, there were an estimated 80,000 deaths and 900,000 hospitalizations from flu during the 2017 to 2018 flu season, making it the worst flu season in at least four decades, according to the Centers for Disease Control and Prevention (CDC).

Causes Of Influenza Virus

Influenza is contagious, which means it can be spread easily from person to another. Viruses that cause influenza spread from person to person mainly by droplets of respiratory fluids sent through the air when someone infected with the virus coughs or sneezes. Other people inhale the airborne virus and can become infected.

Flu virus can live on some surfaces for up to 24 hours. This means that, in some cases, the flu can be spread when someone touches a surface (e.g., doorknobs, countertops, telephones) that has the virus on it and then touches their nose, mouth, or eyes. The flu is most easily spread in crowded places such as schools and offices.

There are three families of influenza virus: A, B, and C

Influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States, while influenza C causes only mild respiratory symptoms and is not thought to cause epidemics, according to the CDC. Type C more commonly affects ducks, geese, turkeys, and chickens, but it has also been involved in a small percentage of human cases, most of which are in children. Type B mainly affects humans and usually causes a milder disease, and it changes very little from year to year.

Type A influenza poses the most serious problems for humans and causes 95% of cases. Strains of this type have also been found in birds, horses, pigs, seals, whales, and ferrets. Viruses that affect two different species sometimes combine and mix-and-match genetic information to create a new strain that nobody is immune to and for which no vaccine has been prepared.

The flu takes 1 to 4 days to incubate in humans, but infected people can become contagious starting the day before symptoms appear. Adults remain infectious (i.e., they can spread the virus to others) for about 6 days, and children remain infectious for up to 10 days.

While there are many types of flu, it is important to note that the “stomach flu” isn’t actually a type of influenza. It is actually gastroenteritis, an inflammation of the lining of the intestines caused by a virus, bacteria or parasites.

Also check: Flu Shot Facts & Side Effects (Updated for 2018-2019)

In 2018, researchers also confirmed that the virus can spread just by breathing, through small particles called aerosols. Other research has found that such infectious particles can travel up to six feet after they are exhaled by a sick person.

Symptoms Of Influenza Virus

People often get the symptoms of the common cold and the flu mixed up. While it can be hard to tell the difference between a cold and the flu, in general, flu symptoms tend to be more severe than cold symptoms, according to the CDC. Initial flu symptoms include headaches, chills, and a cough. Symptoms such as fever, loss of appetite, and muscle aches soon follow. Other symptoms such as nausea, vomiting, and diarrhea are rare in adults but more common in children. Here are some more common signs and symptoms of the flu, according to the Mayo Clinic:

  • Headache
  • Aching muscles, especially in your back, arms and legs
  • Fever.
  • Chills and sweats.
  • Sore throat.
  • Dry, persistent cough.
  • Weakness.
  • Nasal congestion.


Most people infected with influenza virus recover within one to two weeks without requiring medical treatment, according to the WHO. “It is very important for anyone diagnosed with influenza to take care of themselves, giving themselves enough time, enough fluids and enough rest to fully recover,” said Dr. Susan Donelan, medical director of health care epidemiology at Stony Brook University in New York.

Over-the-counter painkillers such as ibuprofen (Advil, Motrin) and aspirin may help reduce fevers and relieve aches and pains during the flu. Decongestant drops and cough syrups may also help ease symptoms, but always contact a medical professional before administering over-the-counter remedies to children.

Certain people are at greater risk for health complications from the flu that could result in hospitalization or death. This includes people older than 65, children under 5, pregnant women and people with certain health conditions, such as heart disease, asthma, kidney disease and diabetes.

According to the American College of Emergency Physicians, signs that the flu requires emergency care for adults include:

• Difficulty breathing or shortness of breath.
• Chest pain or abdominal pain.
• Sudden dizziness.
• Confusion.
• Severe or persistent vomiting.
• Flu-like symptoms that appear to get better, but then return with a fever and worse cough.
• Swelling in the mouth or throat.

In children, emergency symptoms include:

• Fast breathing or trouble breathing.
• Bluish skin color.
• Not drinking enough fluids.
• Not waking up or not interacting.
• Being so irritable that the child does not want to be held.
• Flu-like symptoms that improve, but then return with a fever and worse cough.
• Fever with a rash.

At the hospital, the doctor may administer antiviral drugs, including adamantanes, such as amantadine and rimantadine (Flumadine), and inhibitors of influenza, including neuraminidase inhibitors, such as oseltamivir (Tamiflu) and zanamivir (Relenza), or Peramivir and laninamivir (Inavir), if the patient is seen within 48 hours of onset of symptoms, according to WHO. The CDC recommends that high-risk patients with a flu-like illness get prompt treatment with influenza antiviral drugs, without waiting for testing results to confirm the flu.


The CDC recommends a yearly flu vaccine for everyone 6 months and older. “For the seasonal flu, those who are younger, those who are older, and those who are immunocompromised are more likely to contract influenza; and if someone in that group is unable to get vaccinated, it is important for those who have close interaction with them or care for them to get vaccinated to reduce their exposure,” said Donelan.

So, why do some people still get the flu after getting a flu shot? The flu vaccine helps protect against the viruses that are predicted to be most common for that particular year. But it is possible to contract a strain of the virus that is slightly different from those included in the seasonal vaccine.

Still, studies show that when strains in the vaccine are a good match with the ones that are circulating, vaccinated individuals are 60 percent less likely to catch the flu than people who aren’t vaccinated, according to the CDC.

Additional resources

CDC: Preventing Seasonal Flu with Vaccination
National Foundation for Infectious Diseases: Influenza
U.S. Food and Drug Administration: Influenza Virus Vaccine Safety & Availability