The Flu (Influenza): 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

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).

Many people originally called an epidemic "visitation, influence (of the stars)," from the Medieval Latin word influential. Italians also used the word influenza for diseases since at least 1504 because they, like those that used the Latin word influential, believed that the stars influenced health, according to the Douglas Harper Etymology Dictionary. Influenza in Italian literally means “influence.” The Italians would add the word influenza to the names of ailments. For example, influenza di febbre scarlattina was "scarlet fever."

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 U.S. Department of Health & Human Services. 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. A new form of influenza, called H1N1, appeared. This virus is also called swine flu because the virus is similar to a virus found in pigs. It is not called swine flu because it can be contracted from pigs or by eating pork.

The first U.S. case of H1N1 was diagnosed on April 15, 2009. People in 74 countries were affected and 43 million to 89 million people had H1N1 between April 2009 and April 2010. The Centers for Disease Control and Prevention (CDC) estimated there were between 8,870 and 18,300 H1N1 related deaths during this time.

The main three types of influenza virus are named A, B, and C. A and B viruses cause seasonal epidemics of disease almost every winter in the United States while C causes only mild respiratory symptoms, according to the Centers for Disease Control and Prevention (CDC). The 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 650 cases of the bird flu in humans have been reported from 15 countries since 2003, according to the U.S. Department of Health & Human Services. It is contracted directly from birds and is not spread from human to human like most types of influenza. 

The influenza virus is very contagious and is spread from person to person by droplets and small particles from the cough or sneeze of an infected person. People older than 65, children under 5 — especially those under 2 years — pregnant women, the obese and people with certain health conditions, such as heart disease, asthma, kidney disease and diabetes, are more at risk of health complications from the flu that could result in hospitalization or death.

Are there some cases where a seemingly healthy person may die of the flu? The Spanish flu killed many healthy adults ages 20 to 50 years old. The reason is still unknown.

"There are some thoughts that a very robust immune system can have an overly exuberant inflammatory response to influenza, that could be potentially detrimental to the patient," Dr. Susan Donelan, medical director of health care epidemiology at Stony Brook Medicine.

People often get the symptoms of the common cold and the flu mixed up. One of the biggest differences between the two is the accompanying fever. The common cold is usually accompanied by a low-grade fever, while those with the flu often experience a fever of over 100 degrees Fahrenheit (38 degrees Celsius). Colds rarely cause a fever or headaches, either, and the flu almost never causes an upset stomach, according to U.S. National Library of Medicine

Here are some more common signs and symptoms of the flu include, 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 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 Donelan.

Over-the-counter painkillers such as ibuprofen (Advil, Motrin) and aspirin are often taken to reduce fevers and help 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. 

According to the Mayo Clinic, adults should see a medical professional if:

  • their fever rises to 103 F (39.4 C) or higher
  • the fever accompanied by sweating
  • they experience chills and a cough with colored phlegm
  • they have significantly swollen glands
  • feel severe sinus pain 

Children should be taken to see a medical professional if they experience: 

  • fever of 100.4 F (38 C) in newborns up to 12 weeks
  • fever that rises repeatedly above 104 F (40 C) in a child of any age
  • fever that lasts more than 24 hours in a child younger than 2
  • fever that lasts more than three days in a child older than 2
  • drinking inadequate fluids or signs of dehydration, such as urinating less often than usual
  • difficulty breathing
  • ear pain
  • persistent cough
  • severe headache
  • vomiting or abdominal pain
  • unusual sleepiness
  • stiff neck
  • persistent crying

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.

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. The CDC also recommends that high-risk patients with a flu-like illness get prompt treatment with influenza antiviral drugs without waiting for confirmatory testing.

"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.

Why do some people still get the flu after getting a flu shot? The flu vaccine protects against the viruses that will be most common for that particular year, but not all types of the virus. It is possible to contract a strain of the virus that is not included in that year's vaccine. 

Additional resources