What Is Flu? Symptoms, Causes, Treatment, and Medications

Flu is caused by influenza viruses that infect the respiratory tract. The main types that cause seasonal epidemics in people are influenza A and influenza B.

How flu spreads:

• Through respiratory droplets when an infected person coughs, sneezes, or talks
• By touching a surface or object with flu virus on it and then touching the mouth, nose, or eyes
• Most easily in the first few days of illness, though some people can spread it before symptoms start

Factors that raise the risk of catching or spreading flu:

• Close contact with infected people, such as in households, schools, and workplaces
• Crowded indoor settings, especially in fall and winter
• Not being vaccinated
• Touching the face with unwashed hands

Groups at higher risk of serious flu complications:

• Adults 65 years and older
• Young children, especially under 5
• Pregnant people
• People with chronic conditions such as asthma, diabetes, heart disease, or a weakened immune system

Flu symptoms often come on suddenly and are more intense than a cold. Not everyone with flu has a fever.

Common flu symptoms include:

• Fever or feeling feverish and chills
• Cough
• Sore throat
• Runny or stuffy nose
• Muscle or body aches
• Headache
• Fatigue (tiredness)
• Some people, more often children, may have vomiting and diarrhea

How Long Do Flu Symptoms Last?

Most people with the flu recover in a few days to less than two weeks:

• Acute symptoms: usually last about 3-7 days
• Cough and fatigue: can linger for two weeks or more
• Complications: some people develop pneumonia or other serious problems that require medical care

Seek care right away if symptoms improve and then return with a worse fever or cough, which can be a sign of a secondary infection.

Doctors can often diagnose flu based on symptoms, especially during flu season when the virus is widespread in the community. Testing may be used to confirm the diagnosis or guide treatment.

A healthcare provider may ask:

• When did your symptoms start and how quickly did they come on?
• Do you have a fever, cough, body aches, or fatigue?
• Have you been around anyone with the flu?
• Are you in a high-risk group or do you have chronic health conditions?

Diagnostic tests may include:

• Rapid influenza diagnostic tests, which give results in about 15-30 minutes
• Molecular tests such as RT-PCR, which are more accurate and can identify the virus type
• A nasal or throat swab to collect the sample

Testing is especially useful for people who are hospitalized, at high risk, or when it will change treatment decisions.

Most healthy people with the flu recover with rest and home care. Antiviral medications can shorten illness and reduce complications, especially when started early.

Antiviral Medications

The CDC recommends four FDA-approved antiviral drugs to treat flu:

• Oseltamivir (Tamiflu), an oral neuraminidase inhibitor and the most commonly prescribed option
• Baloxavir marboxil (Xofluza), a cap-dependent endonuclease inhibitor given as a single oral dose
• Zanamivir (Relenza), an inhaled neuraminidase inhibitor
• Peramivir (Rapivab), given by IV, often for hospitalized patients

Antivirals work best when started within about 2 days of symptom onset. They can make illness milder, shorten how long you are sick, and may help prevent complications such as pneumonia.

Home Care and Symptom Relief

• Rest and drink plenty of fluids
• Over-the-counter fever and pain relievers such as acetaminophen or ibuprofen
• Cough and congestion relief as needed
• Stay home to avoid spreading the virus to others

Note: Children and teenagers with flu should not take aspirin because of the risk of Reye syndrome. Antibiotics do not treat the flu, since it is caused by a virus, not bacteria.

Most people recover from the flu without problems, but it can cause serious complications, especially in high-risk groups.

Possible complications include:

• Pneumonia (viral or a secondary bacterial infection)
• Bronchitis and sinus or ear infections
• Worsening of chronic conditions such as asthma, heart failure, or diabetes
• Dehydration
• Inflammation of the heart (myocarditis), brain (encephalitis), or muscles (myositis)
• Sepsis, a life-threatening response to infection

People at higher risk for complications include older adults, young children, pregnant people, and those with chronic health conditions or weakened immune systems. Seek care quickly if warning signs such as trouble breathing or chest pain develop.

The single best way to prevent the flu is to get a flu vaccine every year. The CDC recommends annual vaccination for everyone 6 months and older, with rare exceptions.

Prevention steps:

• Get an annual flu vaccine, ideally before flu season ramps up in the fall
• Wash your hands often with soap and water, or use hand sanitizer
• Avoid touching your eyes, nose, and mouth
• Cover coughs and sneezes
• Avoid close contact with people who are sick
• Clean and disinfect frequently touched surfaces
• Stay home when you are sick to avoid spreading the virus

In some cases, antiviral medications may be used to help prevent flu after a known exposure, especially in high-risk people. The vaccine takes about two weeks to build protection, so earlier is better.

Most people with the flu do not need to see a doctor. However, high-risk people should contact a provider early, because antiviral treatment works best when started quickly.

Contact a healthcare provider if you:

• Are in a high-risk group (older adults, young children, pregnant people, or those with chronic conditions)
• Have symptoms that are severe or not improving
• Have a high fever that will not come down

Seek emergency care (call 911 in the U.S.) for warning signs such as:

• Difficulty breathing or shortness of breath
• Persistent pain or pressure in the chest or abdomen
• Persistent dizziness, confusion, or trouble waking up
• Seizures
• Not urinating or signs of severe dehydration
• Severe muscle pain or weakness
• Symptoms that improve then return with worse fever or cough

In children, also watch for fast or troubled breathing, bluish lips or face, ribs pulling in with each breath, and not being alert or interactive when awake.

Over-the-counter (OTC) medications relieve flu symptoms but do not fight the virus itself. Antiviral drugs, which target the virus, require a prescription.

OTC options for symptom relief:

• Acetaminophen (Tylenol) or ibuprofen (Advil) for fever, aches, and headache
• Cough suppressants such as dextromethorphan (Delsym)
• Decongestants and saline nasal sprays for congestion
• Throat lozenges for sore throat

You should see a doctor for prescription antivirals if you:

• Are at high risk for flu complications
• Have severe symptoms or are getting worse
• Want treatment that can shorten the illness, ideally started within 2 days of symptom onset

Prescription antivirals include oseltamivir (Tamiflu), baloxavir marboxil (Xofluza), zanamivir (Relenza), and peramivir (Rapivab). The annual flu vaccine, which prevents illness, is also administered by a healthcare professional or pharmacist. Remember, children and teens should not take aspirin for flu.