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The Flu (Influenza): Causes, Symptoms & Treatment

influenza, bird flu mutation
This digitally-colorized negative-stained transmission electron micograph (TEM) shows a number of influenza A viruses. H1N1 is a strain of influenza A. (Image credit: CDC/ F.A. Murphy)

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.

For most people, the flu is an inconvenience that subsides in a few days. For others, influenza can lead to health complications, visits to the hospital and even death. 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).


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. 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).

The swine flu pandemic caused an estimated 43 million to 89 million illnesses in the United States between April 2009 and April 2010. The CDC estimated there were between 8,870 and 18,300 H1N1-related deaths during this time.


The main three types of influenza virus that cause illness in people are named 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. The influenza A virus is broken down into subtypes, and both A and B are broken down into strains for classification.

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, avian influenza (bird flu, H5N1) is a flu virus that typically only affects birds. It is very rare for a human to contract it, and only around 700 cases of this bird flu in humans have been reported from 15 countries since 2003, according to the CDC. It is most often contracted directly from birds and is usually not spread from human to human like most types of influenza.

Another type of rare bird flu, called H7N9, first appeared in people in China in 2013. Since then, the virus has caused several hundred human infections per year in China; but there was spike in cases from 2016 to 2017, when 766 human cases in China were reported. The H7N9 virus also does not appear to spread easily between people.

In contrast to bird flus, influenza A and B viruses are very contagious, and can spread from person to person by droplets from the cough or sneeze of an infected person. 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.


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. For example, people with the flu usually develop a fever, whereas people with colds rarely do. 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 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.

In some cases, even seemingly healthy people can become severely ill from the flu. For example, the Spanish flu killed many otherwise healthy adults ages 20 to 50 years old. The reason for the high death rate in young adults is still unknown.

According to the American College of Emergency Physicians, signs that the flu requires emergency care 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..


Though washing your hands regularly and practicing good hygiene are good tactics for preventing the flu, the best course of action is to receive the flu vaccine every year. Each year, researchers determine what strain of the influenza virus will be most active and vaccines are produced to prevent infection.

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.

And, a 2013 study found that those who do get sick may be less likely to develop less serve symptoms if they are vaccinated.

Additional resources

This article was updated on Oct. 2, 2018 by Live Science Senior Writer, Rachael Rettner. 

Alina Bradford
Alina Bradford
Alina Bradford is a contributing writer for Live Science. Over the past 16 years, Alina has covered everything from Ebola to androids while writing health, science and tech articles for major publications. She has multiple health, safety and lifesaving certifications from Oklahoma State University. Alina's goal in life is to try as many experiences as possible. To date, she has been a volunteer firefighter, a dispatcher, substitute teacher, artist, janitor, children's book author, pizza maker, event coordinator and much more.