The flu (short for influenza) is a viral respiratory disease that affects the throat, nose and lungs. There are different types of influenza viruses, and they evolve 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, hospital visits and even death. Globally, there are an estimated 1 billion cases of flu each year, and 3 million to 5 million of these cases are severe, leading to about 290,000 to 650,000 deaths annually, according to the World Health Organization (WHO).

In the United States, there were an estimated 35 million cases of flu, 380,000 flu-related hospitalizations and 20,000 flu-related deaths during the 2019-2020 flu season (the last flu season with data available before the COVID-19 pandemic), according to the Centers for Disease Control and Prevention (CDC).

History

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 influenza pandemic, known as "The Great Pandemic," is estimated to have infected one-third of the worldwide population and caused 50 million deaths, according to the CDC. This pandemic was also named the "Spanish flu" because early reports of flu cases during that pandemic came from Spain.

A more recent flu 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 one found in pigs.

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 that time.

Causes

The three main 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. A fourth type of influenza, type D, is not known to cause illness in people and mainly affects cattle, according to the CDC.

The influenza A virus is broken down into subtypes based on two proteins on the virus's surface called hemagglutinin (H) and neuraminidase (N). A total of 18 different hemagglutinin proteins and 11 different neuraminidase proteins are known to exist, and these two proteins appear together in various combinations, according to the CDC. Over 130 influenza A subtype combinations have been identified in nature, with most infecting wild birds. The two subtypes of influenza A that commonly cause seasonal flu in people are called H1N1 and H3N2. Influenza B viruses aren't divided into subtypes; instead, they are classified into two lineages: B/Yamagata and B/Victoria.

While there are many types of flu, it is important to note that the "stomach flu" isn't a type of influenza. Rather, it is gastroenteritis, an inflammation of the lining of the intestines caused by a virus (such as norovirus or rotavirus), bacteria or parasites.

Also, avian influenza, or bird flu, refers to flu viruses that typically affect only birds. That's because bird flu viruses are adapted to birds and don't easily spread to other animals, including humans. But in rare cases, certain strains of bird flu have been known to infect humans. These strains include H5N1, H7N9, H5N6, H5N8 and, most recently, H3N8, which was reported in humans for the first time in April 2022, in a 4-year-old boy in China, Live Science previously reported. Among these bird flu strains, H5N1 is perhaps the best known; around 700 cases of H5N1 have been reported worldwide in humans since 2003, according to the CDC. It is most often contracted directly from birds and, unlike most types of influenza, is usually not spread from person to person.

How flu spreads

Flu viruses spread from person to person, mainly by droplets from a cough or sneeze of an infected person, according to the CDC. In 2018, researchers also confirmed that the virus can spread through small particles called aerosols that are released when a person breathes, Live Science previously reported. Other research has found that these infectious particles can travel up to 6 feet (1.8 meters) after they are exhaled by a sick person. Less commonly, people may become infected when they touch surfaces contaminated with flu virus particles and then touch their mouth, nose or eyes.

People with the flu may be contagious starting one day before their symptoms begin and up to seven days after becoming sick, though they are most contagious in the first three to four days after their symptoms begin, according to the CDC. Young children and people with weakened immune systems may be contagious for longer periods. In addition, some people may not develop symptoms but still spread the virus to others (i.e. asymptomatic spread), according to the CDC.

Flu symptoms

People often confuse the symptoms of flu with those of the common cold. While it sometimes 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 common signs and symptoms of the flu, according to the CDC:

Fever/chillsCoughSore throatRunny or stuffy noseMuscle or body achesHeadacheFatigueVomiting/diarrhea (more common in children than in adults)

Flu complications

Most young and healthy people don't develop serious complications from the flu. But serious effects can occur, particularly among people in high-risk groups, including young children, older adults, pregnant people and those with certain chronic medical conditions. According to the National Institutes of Health, flu can have the following complications:

Pneumonia: An infection of the lungs, or pneumonia, is a serious complication of the flu. It can potentially be deadly for older adults and other high-risk individuals.Bronchitis: Flu is a common cause of acute bronchitis, or inflammation of airways in the lungs known as bronchial tubes.Worsening asthma: People with asthma may have flare-ups when they have the flu.Inflammation: The flu may trigger inflammation of the heart (myocarditis), brain (encephalitis) or muscle tissues (myositis or rhabdomyolysis).Ear infections: Young children are more susceptible to this complication.

Flu treatment

Most people infected with influenza recover within a few days to less than two weeks without requiring medical treatment, according to the CDC. "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 pain relievers, such as ibuprofen and aspirin, may help reduce fevers and relieve aches and pains during the flu. Decongestant drops and cough syrups also may 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. These groups include people older than 65, children under 5, pregnant people, and people with certain health conditions, such as heart disease, asthma, kidney disease and diabetes, according to the CDC. If you fall into a high-risk group and develop flu symptoms, you should contact your health care provider promptly because you may benefit from taking antiviral drugs to treat the flu. The CDC recommends that people at high risk for flu complications receive antiviral drugs as soon as possible, since these drugs work best if given within two days from symptom onset, according to the agency.

Antiviral drugs are prescription medications that fight flu viruses and can shorten the duration of illness and lower the risk of flu complications. Antiviral drugs approved by the Food and Drug Administration include oseltamivir phosphate (Tamiflu), zanamivir (Relenza), peramivir (Rapivab) and baloxavir marboxil (Xofluza), according to the CDC. The agency 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.

People with the flu don't need to go to the emergency room unless they have emergency symptoms of the flu, which, according to the CDC, include the following:

Difficulty breathing or shortness of breathPersistent chest pain or abdominal painPersistent dizziness or confusionSeizuresLack of urinationSevere muscle painSevere weakness or unsteadinessFlu-like symptoms that appear to get better but then return with a fever or worse coughWorsening of chronic medical conditionsIn children, emergency symptoms include the following:

Fast breathing or trouble breathingBluish lips or faceRibs pulling in with each breathChest painSevere muscle pain (for example, child refuses to walk)Dehydration (no urine or wet diapers for 8 hours, no tears when crying, dry mouth)Not alert or interacting when awakeSeizuresFever above 104 degrees Fahrenheit (40 degrees Celsius)Any fever in children younger than 12 weeks old Flu-like symptoms that improve but then return with a fever or worse cough.Worsening of chronic medical conditions

Prevention

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 which strains of the influenza virus are most likely to circulate during flu season, 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," Donelan said.

So, why do some people still get the flu after getting a flu shot? The flu vaccine helps protect against the flu strains that are predicted to be the most prevalent for that particular year. But because the strains included in the vaccine don't always end up matching the strains circulating that year, it is possible to contract a strain of the virus that is different from those included in the seasonal vaccine. It is also possible to become infected with a strain that is included in the vaccine, although symptoms are usually less severe than they would be if the person had not received the vaccine.

Still, studies show that when strains in the vaccine are a good match with the ones that are circulating, vaccinated individuals are 40% to 60% less likely to catch the flu than people who aren't vaccinated, according to the CDC.

Additional resources

Search for flu vaccines near you on Vaccines.gov. Read more about the differences between cold and flu on the CDC's "cold versus flu" page. The CDC also has tips on what to do if you get sick with flu and how to care for someone with flu.

This article was updated on Oct. 12, 2022 by Live Science Contributor Rachael Rettner.