The flu (influenza): Causes, symptoms and treatment

The flu can affect the throat, nose and lungs. The types of flu viruses evolve over time.

The flu is an annoyance that goes away in a few days. Flu can cause health problems for others. According to the World Health Organization, there are an estimated 1 billion cases of flu each year, and 3 million to 5 million of these cases are severe.

According to the Centers for Disease Control and Prevention, there were an estimated 35 million cases of flu, 380,000 flu-related hospitalizations and 20,000 flu-related deaths in the US during the 2019-2020 flu season.

History

Since at least the 1500s, Italians have used the word "influenza" to mean "influence" because they believed that the stars influenced health. The flu has caused many major epidemics in the past. The Great Pandemic of 1918 to 1919 caused 50 million deaths and one-third of the world's population, according to the Centers for Disease Control and Prevention. The Spanish flu was named after the early reports of flu cases in that country.

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There was a new form of the flu strain H1N1 in 2009. It's called "swine flu" because it's similar to a pig's virus.

An estimated 43 million to 89 million illnesses were caused by the swine flu in the United States. Between 8,870 and 18,300 deaths were related to H1N1 during that time, according to the CDC.

Causes

The three main types of flu are A, B and C. According to the CDC, a fourth type of flu, called type D, is not known to cause illness in people.

The hemagglutinin (H) and neuraminidase (N) are the two main components of the flu A virus. According to the CDC, there are 18 different hemagglutinin and 11 different neuraminidases that are known to exist. Most of the infecting birds are wild birds. H1N1 and H3N2 are the two most common types of flu that people get.

The "stomach flu" is not a type of flu. There is an inflammation of the lining of the intestines caused by a virus.

Bird flu is a type of flu that usually affects birds. Bird flu viruses are adapted to birds so they don't spread to other animals. Certain strains of bird flu have been known to cause illness. H5N1, H7N9, H5N6, H5N8 and, most recently, H3N8, have been reported in humans for the first time. Around 700 cases of H5N1 have been reported worldwide in humans since 2003 according to the CDC. Unlike most types of flu, it isn't spread from person to person.

How flu spreads

According to the CDC, flu Viruses spread from person to person by droplets from a cough or sneeze. The virus can be spread through small particles called aerosols that are released when a person breathes. After a sick person exhales infectious particles, they can travel up to six feet. People may become sick if they touch surfaces that are contaminated with flu virus particles and then touch their mouths, noses, or eyes.

The CDC says that people with the flu are most contagious in the first three to four days after their symptoms start. People with weakened immune systems are more likely to be contagious. According to the CDC, some people don't have symptoms but still spread the virus to others.

Flu symptoms

People think the symptoms of flu are the same as those of the cold. According to the CDC, flu symptoms tend to be more severe than cold symptoms. People with the flu are more likely to develop a cold than people with a cold. According to the CDC, there are some signs of the flu.

• Fever/chills
• Cough
• Sore throat
• Runny or stuffy nose
• Muscle or body aches
• Headache
• Fatigue
• Vomiting/diarrhea (more common in children than in adults)

Flu complications

Young and healthy people don't get sick from the flu. Among people in high-risk groups include young children, older adults, pregnant people and those with chronic medical conditions. According to the National Institute of Health, flu can have a number of serious consequences.

• 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

According to the CDC, most people recover from the flu in a few days to less than two weeks. It's important for anyone diagnosed with the flu to take care of themselves, giving themselves enough time, fluids and rest to fully recover.

Ibuprofen and aspirin can be used over-the-counter to help reduce the effects of the flu. Contact a medical professional before giving over-the-counter remedies to children.

Some people are more at risk for health problems from the flu than others. People older than 65, children under 5, pregnant people, and people with certain health conditions are included in these groups. If you fall into a high-risk group and become sick with the flu, you should contact your health care provider as soon as possible. If you're at high risk for the flu, the CDC recommends that you get the drugs as soon as possible, since they work best if given within two days of symptom onset.

Anti-flu drugs can shorten the duration of illness and reduce the risk of flu-related problems. According to the CDC, the Food and Drug Administration has approved several anti-viral drugs. The agency recommends that high-risk patients with a flu-like illness get treatment without waiting for test results to confirm the flu.

The CDC says that people with the flu don't need to go to the emergency room if they have emergency symptoms.

• Difficulty breathing or shortness of breath
• Persistent chest pain or abdominal pain
• Persistent dizziness or confusion
• Seizures
• Lack of urination
• Severe muscle pain
• Severe weakness or unsteadiness
• Flu-like symptoms that appear to get better but then return with a fever or worse cough
• Worsening of chronic medical conditions

Emergency symptoms can be seen in children.

• Fast breathing or trouble breathing
• Bluish lips or face
• Ribs pulling in with each breath
• Chest pain
• Severe 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 awake
• Seizures
• Fever 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

The best way to prevent the flu is to get the flu vaccine every year. The strains of the flu virus that are most likely to circulate in a given year are determined by researchers.

Everyone should get a yearly flu vaccine, according to the CDC. Those who are younger, those who are older, and those who are immunocompromised are more likely to contract the flu, so it's important for those who have close interaction with them or care for them.

Some people get the flu after getting a vaccine. The flu vaccine helps protect against the most common flu strains. It is possible to contract a strain of the virus that is different from the one included in the seasonal vaccine because the strains included in the vaccine don't always match. If a person hasn't received the vaccine, they can be exposed to a strain that is included in the vaccine.

According to the CDC, people who get the vaccine are 40% to 60% less likely to get the flu than people who don't.

Additional resources

You can find the flu vaccine near you on Vaccines.gov. There are differences between cold and flu on the CDC's website. If you get sick with the flu, the CDC has tips on what to do.

The article was updated in October of 2022.