Flu (Influenza)

Flu symptoms

Flu usually hits suddenly, often within a few hours. Typical symptoms are:

• fever of 38 °C (100.4 °F) or higher, with chills or sweats (not everyone with flu has a fever)
• muscle and body aches
• headache
• dry cough
• sore throat
• runny or stuffy nose
• severe tiredness and weakness
• vomiting and diarrhea, more common in children

Older adults may have fewer classic symptoms, presenting instead with weakness, confusion or worsening of chronic illness.

What "flu-like symptoms" and "flu syndrome" mean

Doctors use flu-like syndrome or influenza-like illness to describe a cluster of fever, aches, chills, headache, tiredness and cough or sore throat, whatever the cause. Influenza is the classic cause, but many other infections and conditions can produce the same picture:

• COVID-19 and RSV
• other respiratory viruses (adenovirus, parainfluenza, rhinovirus)
• early stages of hepatitis, mononucleosis or HIV infection
• tick-borne illnesses such as Lyme disease and ehrlichiosis
• urinary tract and kidney infections
• malaria and dengue in travelers
• vaccine reactions for 1 to 2 days after vaccination
• some medicines, such as interferon and zoledronic acid infusions

Flu-like symptoms that come back in a regular pattern, for example monthly, can be caused by periodic fever syndromes, premenstrual changes, or monthly medicines or infusions. These need medical review. See flu-like syndrome for more.

Flu vs cold vs COVID-19

Because symptoms overlap, testing is the only reliable way to tell flu from COVID-19.

How flu spreads

Flu spreads mainly through droplets when infected people cough, sneeze or talk, and less often by touching contaminated surfaces and then the mouth, nose or eyes. People are most contagious in the first 3 to 4 days of illness, and can spread it from 1 day before symptoms start to about 5 to 7 days after. Children and people with weakened immunity can spread it longer.

Who is at higher risk of complications

• adults 65 and older
• children under 5, especially under 2
• pregnant people and those up to 2 weeks after delivery
• people with asthma, COPD, heart disease, diabetes, kidney or liver disease, or neurological conditions
• people with weakened immune systems
• people with obesity (BMI 40 or higher)
• residents of nursing homes

Complications

• pneumonia, viral or bacterial
• worsening of asthma, COPD and heart failure
• ear and sinus infections
• myocarditis (heart inflammation) and heart attack
• encephalitis and other brain complications
• muscle inflammation and breakdown
• sepsis and multi-organ failure

Diagnosis

Flu is often diagnosed from symptoms during flu season. Tests, from a nose or throat swab, are used when the result would change treatment, such as in hospitalized or high-risk patients:

• rapid molecular tests, which give results in about 15 to 30 minutes
• rapid antigen tests, which are less sensitive
• combined flu and COVID-19 tests, including some home tests

Treatment

Home care

• rest and stay home until you have been fever-free for at least 24 hours without fever-reducing medicine and symptoms are improving
• drink plenty of fluids
• use acetaminophen (paracetamol) or ibuprofen for fever and aches, following the label dose. Do not give aspirin to children or teenagers, because of the risk of Reye's syndrome
• use saline nose sprays, honey for cough in people over 1 year, and a humidifier

Antiviral medicines

Prescription antivirals shorten illness by about a day and reduce the risk of complications, particularly when started within 48 hours of symptom onset. Options include oseltamivir, baloxavir, zanamivir and peramivir. They are recommended for:

• anyone hospitalized with flu
• anyone with severe or worsening illness
• people at high risk of complications, even if started after 48 hours

Healthy people with mild illness may also be offered them if seen early.

Antibiotics do not treat flu but are used for bacterial complications such as pneumonia.

Prevention

• Annual flu vaccine for everyone 6 months and older. It reduces the risk of illness, hospitalization and death. The vaccine is updated each year to match circulating strains.
• Wash hands often and cover coughs and sneezes.
• Stay home when sick and keep sick children home.
• Antiviral prevention may be offered to high-risk people after close exposure, such as during outbreaks in nursing homes.

Frequently asked questions

What are flu-like symptoms?

Flu-like symptoms are fever, chills, muscle aches, headache, fatigue and often a cough or sore throat. Influenza is the classic cause, but COVID-19, other viruses, some bacterial and tick-borne infections, and certain medicines can cause the same pattern.

How long does the flu last?

Most people feel better within 3 to 7 days, but a cough and tiredness can last 2 weeks or longer. Older adults and people with chronic illness may take longer to recover. Symptoms that worsen after initial improvement need medical review.

When is the flu contagious?

You can spread flu from about 1 day before symptoms start until 5 to 7 days after becoming sick. You are most contagious in the first 3 to 4 days. Stay home until you are fever-free for at least 24 hours without medicine.

Should I take Tamiflu for the flu?

Antivirals such as oseltamivir (Tamiflu) are most effective when started within 48 hours. They are recommended for people at high risk of complications, those who are hospitalized and those with severe illness. Ask your doctor promptly if you think you have flu and are in a high-risk group.

Can you get the flu even if you had the flu shot?

Yes, but the vaccine reduces the chance of getting sick and makes illness milder if you do catch it. It also lowers the risk of hospitalization and death. Effectiveness varies each year depending on how well it matches circulating strains.