Overview
The flu, also called influenza, is an infection of the nose, throat, and lungs, which are part of the respiratory system. The flu is caused by a virus. Influenza viruses are different from the viruses that cause gastroenteritis, which cause diarrhea and vomiting.
Most people who get the flu recover on their own. But sometimes this illness and its complications can be life-threatening. To protect yourself against seasonal influenza, you can get a flu vaccine every year. Although the vaccine isn’t 100% effective, it reduces the chances of developing serious complications from the infection. This is especially important for people at high risk of flu complications.
In addition to the vaccine, you can take other steps to prevent flu infection. You can clean and disinfect surfaces, wash your hands, and keep the air around you moving.
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Symptoms
The viruses that cause influenza spread at high levels during certain times of the year in the Northern and Southern Hemispheres. These periods are called flu season. In North America, flu season typically occurs between October and May.
Flu symptoms, such as a sore throat and a stuffy or runny nose, are common. You can also experience these symptoms with other illnesses, such as a cold. But colds usually start slowly, while the flu typically comes on quickly, two or three days after exposure to the virus. And while a cold can be a nuisance, you generally feel much worse with the flu.
Other common flu symptoms include the following:
- Fever
- Cough
- Headache
- Muscle pain
- Feeling very tired
- Sweating and chills
In children, these symptoms may manifest more generally as restlessness or irritability. Children are also more likely than adults to experience earaches, stomach upset, vomiting, or diarrhea with the flu.
In some cases, people experience eye pain, watery eyes, or notice that light hurts their eyes.
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When should you consult a doctor?
Most people with the flu can treat the condition at home and often do not need to see a healthcare professional.
If you have flu-like symptoms and are at risk of complications, see a healthcare professional immediately. Taking antiviral medication within two days of symptom onset can shorten the duration of the illness and prevent more serious problems.
If you have flu-like symptoms that indicate a medical emergency, seek immediate medical attention. In adults, symptoms that indicate an emergency may include the following:
- Difficulty breathing or shortness of breath.
- Chest pain or pressure.
- Constant dizziness.
- Difficulty waking up or confusion.
- Dehydration.
- Seizures.
- Worsening of pre-existing diseases.
- Great weakness or intense muscle pain.
In children, symptoms that indicate an emergency include all the symptoms observed in adults, plus the following:
- Rapid breathing or ribs sinking with each breath.
- Bluish or grayish lips or nail beds.
- Absence of tears when crying or dry mouth, in addition to the lack of need to urinate.
- Symptoms, such as fever or cough, that improve, but then return or worsen.
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Causes
The flu is caused by viruses. These viruses travel through the air in droplets when someone with the infection coughs, sneezes, or talks. You can inhale these droplets directly. You can also come into contact with the viruses through an object, such as a computer keyboard, and then transfer them to your eyes, nose, or mouth.
It is possible to spread the virus to others from approximately the day before symptoms appear until five to seven days after they begin. This is called being a contagious carrier. Children and people with weakened immune systems may be contagious for a slightly longer time.
Influenza viruses are constantly changing and new strains often appear.
A person’s first flu infection provides some long-term protection against similar flu strains. But the vaccines offered each year are made to match the flu virus strains most likely to circulate that season. The protection these vaccines offer lasts for months in most people.
Risk factors
There are a number of factors that can increase the risk of contracting the flu virus or suffering complications from a flu infection.
Older and younger age
Seasonal influenza tends to have worse outcomes in young children, particularly those under 2 years old. Adults over 65 also tend to experience worse outcomes.
Living conditions
People who live in facilities with many other residents, such as nursing homes and convalescent homes, are more likely to catch the flu.
Weakened immune system
An immune system that doesn’t quickly clear the flu virus can increase the risk of getting the flu or developing complications from it. People can have a weakened immune response from birth, due to an illness, or as a result of treatment for an illness or medication.
Chronic diseases
Chronic health conditions can increase the risk of developing flu-related complications. Examples include asthma and other lung diseases, diabetes, heart and nervous system diseases, a history of stroke, metabolic disorders, airway problems, and kidney, liver, or blood diseases.
Race or ethnic group
In the United States, American Indians, Alaska Natives, Black people, and Latinos may be at higher risk of requiring hospitalization for medical care due to the flu.
Aspirin treatment
Young people on long-term aspirin treatment are at risk of developing Reye’s syndrome if they become infected with the flu virus.
Pregnancy
People who are pregnant are more likely to experience complications from the flu, particularly in the second and third trimesters.
Obesity
People with a body mass index (BMI) of 40 or higher have a greater risk of developing complications from the flu.
Complications
If you’re young and healthy, the flu is usually not serious. Although you might feel very ill while you have it, the flu typically clears up within a week or two without lasting effects.
But high-risk individuals can develop other health problems after the flu, which are known as complications.
Contracting another infection can be a complication of the flu. This includes illnesses such as croup and sinus or ear infections. Lung infections are another complication. Infection of the heart muscle or the lining of the heart can occur after contracting the flu. And in some cases, people may develop an infection of the central nervous system.
Other complications may include the following:
- Acute respiratory failure syndrome.
- Muscle damage, known as rhabdomyolysis, or muscle swelling, known as myositis.
- Toxic shock syndrome.
- Worsening of a chronic illness, such as asthma or kidney disease.
Prevention
The Centers for Disease Control and Prevention (CDC) in the United States recommends that all people 6 months of age and older who do not have a medical reason to avoid vaccination receive the flu vaccine annually.
Getting the flu vaccine reduces the following:
- The risk of contracting the flu. If the vaccine is administered in a later stage of pregnancy, it also helps protect the newborn from the flu.
- The risk of developing severe illness from the flu and needing hospitalization for this reason.
- The risk of death from the flu.
The 2024-2025 seasonal flu vaccines provide protection against the three flu viruses that are expected to be most prevalent this season.
The vaccine is available in injection, jet injection device, and nasal spray form.
For older children and adults, the flu vaccine is usually given in an arm muscle. Younger children may receive the flu vaccine in a thigh muscle.
If you are an adult under 65, you can choose to receive the vaccine using a jet injection device. This device uses a fine jet of liquid at high pressure to penetrate the skin instead of a needle.
The nasal spray is approved for people aged 2 to 49. It is not recommended for some groups, such as the following:
- People who have had a severe allergic reaction to the flu vaccine in the past.
- Pregnant women.
- Young people who take aspirin or a medicine containing salicylate.
- People with weakened immune systems and caregivers or close contacts of people with weakened immune systems.
- Children aged 2 to 4 years who have received a diagnosis of asthma or wheezing in the last 12 months.
- People who recently received an antiviral medication for the flu.
- People with a cerebrospinal fluid leak or the possibility of a leak, such as with a cochlear implant.
Talk to your healthcare team to find out if you should take any precautions with the nasal spray flu vaccine.
High-dose or adjuvanted flu vaccines are also available. These vaccines can help some people avoid needing hospital care due to the flu. People over 65 can receive them. These vaccines are also recommended for people over 18 who have received a solid organ transplant and are taking medications that weaken their immune response.
You can get the flu vaccine if you have an egg allergy.
The first time children aged 6 months to 8 years receive the flu vaccine, they may need two doses at least four weeks apart. After that, they can receive the single annual dose. Consult your child’s healthcare provider.
In addition, consult with your healthcare team before receiving the flu vaccine if you have previously had a severe reaction to this type of vaccine. People who have had Guillain-Barré syndrome should also consult a healthcare professional before receiving the flu vaccine. And if you are not feeling well when you go to get vaccinated, consult with your healthcare team about whether you should postpone the vaccination.
Control the spread of infection
The flu vaccine is not 100% effective. Therefore, it is important to take steps to reduce the risk of spreading the infection, including the following:
- Wash your hands.Wash your hands thoroughly and frequently with soap and water for at least 20 seconds. If soap and water are not available, use an alcohol-based hand sanitizer that contains at least 60% alcohol. Make sure friends and family you are around frequently, especially children, know the importance of handwashing.
- Avoid touching your face.Keeping your hands away from your eyes, nose, and mouth helps keep germs away from those areas.
- Cover your mouth when you cough or sneeze.Cough or sneeze into a tissue or your elbow. Then wash your hands.
- Clean surfaces.Regularly clean frequently touched surfaces to prevent transmission of infection by touching a surface with the virus and then your face.
- Avoid crowds.The flu spreads easily in places where people congregate, such as daycare centers, schools, office buildings, auditoriums, and public transportation. By avoiding crowds during peak flu season, you reduce your chances of getting infected.
Avoid contact with anyone who is sick.
If you are sick, stay home until you feel better and have been fever-free for 24 hours without taking fever-reducing medication. If your fever returns or you start feeling worse, stay away from others until your symptoms improve and you have been fever-free and fever-free for 24 hours. By doing so, you will reduce the chances of infecting others.