Flu Symptoms: Main Signs, Duration and When to Get Help

The main flu symptoms are fever or chills, cough, sore throat, a blocked or runny nose, body aches, headache, and tiredness. They often begin suddenly. Vomiting and diarrhea can also occur, especially in children. You can have influenza without a fever, and symptoms alone cannot confirm the infection.

Influenza is a respiratory infection caused by flu viruses. Influenza symptoms are commonly called flu symptoms. Recognizing the symptoms can help you decide when to rest, contact a healthcare professional, or seek emergency care. The most useful clues are how quickly you became ill, how you feel overall, and whether any warning signs are developing. Source: CDC influenza overview.

Need help now?

Seek emergency medical care for difficulty breathing, persistent chest pain, confusion, seizures, or difficulty waking up. A child with breathing distress, bluish lips, or reduced alertness also needs immediate care. Do not wait for a flu test. Read the warning signs below.

In this guide

Main symptoms and early signs
Symptoms in children and babies
Duration and recovery timeline
Flu, cold, and COVID-19 comparisons
Testing and diagnosis
Emergency warning signs
Treatment and antivirals
Frequently asked questions

What are the main symptoms of the flu?

Flu commonly causes a combination of respiratory symptoms and whole-body discomfort. You might notice a cough alongside chills, aches, a headache, or unusual exhaustion. Having every symptom is not necessary.

Symptom	What it can feel like
Fever or chills	Feeling hot, shivering, or having an elevated temperature. Fever is not universal.
Cough	A cough that may be dry and can continue during recovery.
Body aches	Sore muscles or widespread aching.
Headache	Head pain alongside other signs of illness.
Fatigue	Feeling unusually tired, weak, or unable to manage normal activities.
Sore throat	A painful or irritated throat.
Runny or blocked nose	Nasal drainage or congestion.
Vomiting or diarrhea	Digestive symptoms, which occur more often in children.

The CDC identifies these as typical symptoms. A symptom checklist can help describe your illness, but it cannot tell you which virus caused it. Source: CDC, About Influenza.

Early flu symptoms: What are the first signs?

There is no single first symptom that everyone experiences. The first signs of flu often appear abruptly, with a mix of chills, aches, headache, tiredness, and respiratory symptoms.

You might feel well earlier in the day and then become too unwell to continue your usual activities. The NHS describes flu as appearing quickly, sometimes within hours, compared with the more gradual start of a cold. That difference is a useful clue, but it is not proof of influenza. Source: NHS flu guidance.

How soon do symptoms appear after exposure?

Symptoms usually begin about two days after infection, with a typical range of one to four days. This incubation period is different from the sudden onset of symptoms once illness begins. You generally cannot identify exactly when or where you caught flu from the timing alone. Source: WHO seasonal influenza fact sheet.

When describing possible signs of influenza to a clinic, explain when the first symptoms started as clearly as possible. For example, saying that chills and coughing began yesterday evening is more useful than saying that you have felt sick recently.

Flu symptoms without fever: Can you still have flu?

Yes. Influenza can occur without a fever. This is particularly relevant in older adults and people with weakened immune systems. A normal temperature does not rule out flu. Our guide to febrile and afebrile illness explains these medical terms in more detail.

Some people have cough, fatigue, or other symptoms without becoming feverish. Frail older adults may also show less typical changes, such as reduced appetite or a change in behavior. New confusion needs urgent assessment rather than being dismissed as ordinary tiredness. Source: CDC clinical signs and symptoms.

What are flu symptoms in adults?

Adults often experience cough, body aches, headache, and fatigue, with or without fever. A sore throat or nasal congestion may accompany these symptoms.

Flu can make everyday activities feel difficult. Loss of appetite and trouble sleeping may add to the discomfort. However, trouble breathing, severe weakness, or persistent chest pain should not be treated as an expected part of recovery. Source: NHS.

When describing symptoms to a clinician, include practical details. Explain whether you can drink, walk safely, stay awake normally, and breathe comfortably. These observations give more useful context than a label such as mild flu or bad flu.

Flu symptoms in kids, children, and babies

Children can develop the same respiratory symptoms as adults, while vomiting and diarrhea may also accompany the illness. Young children may struggle to explain how they feel.

Illustration of a parent offering water to a child resting in bed.

Look at changes from your child's usual behavior and activity. The NHS notes that children may appear less active and can also develop ear pain. Tell the clinician what you have observed rather than assuming that every new symptom is caused by flu. Source: NHS.

For babies and young children, pay particular attention to breathing, feeding, wet diapers, and alertness. Contact a healthcare professional promptly if you are concerned. Children younger than five have a higher risk of flu complications, especially those younger than two. Source: CDC higher-risk groups.

A baby younger than 12 weeks with any fever needs medical care right away. Do not wait to see whether other flu symptoms develop.

How long do flu symptoms last?

Uncomplicated flu symptoms usually resolve within three to seven days, although cough and a general feeling of being unwell can last more than two weeks. Recovery varies, especially in older adults and people with chronic lung disease.

The direction of recovery matters as well as the number of days. Gradual improvement is different from feeling better and then developing new fever, worse coughing, or breathing problems. A return or worsening of symptoms needs medical attention. Source: CDC clinical guidance.

A typical flu recovery timeline

Period	What may happen
Before symptoms	The incubation period is usually one to four days after infection.
Start of illness	Several symptoms may develop suddenly, without a fixed order.
First week	Most uncomplicated symptoms resolve within approximately three to seven days.
Later recovery	Cough or low energy can persist after the main illness improves.

Illustration of an adult resting under a blanket and holding a glass of water.

This is a general guide, not a schedule you must follow. There is no reliable day-by-day symptom pattern for every person. Seek advice if you are not improving after about a week or feel increasingly unwell. Source: NHS advice on seeking care.

Flu vs. cold: How do symptoms differ?

Flu tends to start more suddenly and cause stronger aches, chills, and exhaustion. Colds more often begin gradually with nasal and throat symptoms. These are common patterns, not diagnostic rules.

Feature	Flu	Cold
Beginning	Often abrupt	Usually gradual
Fever	Common, but may be absent	Less common
Aches	Often noticeable or strong	Usually milder
Chills	More common	Less common
Energy	Weakness and tiredness are common	Often less affected
Sneezing and congestion	Possible	Common
Cough	Common and sometimes severe	Usually milder

A blocked nose does not exclude flu, and body aches alone do not confirm it. Both illnesses can produce overlapping symptoms. Source: CDC, Cold Versus Flu.

Flu vs. COVID-19: Can you tell the difference?

No. Symptoms alone cannot reliably distinguish flu from COVID-19. Both can cause fever, coughing, fatigue, aches, sore throat, and nasal symptoms.

Flu is caused by influenza viruses, while COVID-19 is caused by SARS-CoV-2. Because the symptoms overlap, testing may help identify the cause and guide treatment. It is also possible to have both infections at the same time. Source: CDC flu and COVID-19 comparison.

Do not use a single symptom, such as a dry cough, to choose a diagnosis. If you have a higher risk of complications, contact a healthcare professional early rather than waiting for the illness to become more recognizable.

How is the flu diagnosed?

A clinician may diagnose flu from your symptoms and clinical circumstances, while laboratory testing can confirm influenza infection. Not everyone needs a test.

Tests differ in how they detect the virus and how accurately they identify infection. Rapid antigen tests can miss some cases, so a negative result does not always rule out flu. Molecular tests detect viral genetic material and are generally more accurate than rapid antigen tests. Some tests check for flu and COVID-19 together. Source: CDC flu diagnosis guidance.

What information should you give your clinician?

When your first symptoms appeared.
Your main symptoms and whether they are improving or worsening.
Any test results, including when you took the test.
Pregnancy, chronic conditions, or immune-suppressing medicines.
Problems with breathing, drinking, urination, or alertness.

Keep a short note on your phone if remembering the sequence is difficult. You do not need to identify the virus yourself before asking for care.

When to see a doctor for flu symptoms

Contact a healthcare professional promptly if you have a higher risk of complications, feel very unwell, or have concerns about your child's illness. Early advice can help determine whether treatment is appropriate.

People at higher risk should seek advice even when symptoms seem manageable. If you are otherwise healthy but are not improving, or your symptoms are becoming more concerning, arrange an assessment. Emergency warning signs require immediate care rather than a routine appointment. Source: CDC, What To Do If You Get Sick.

Flu warning signs: When is it an emergency?

Get medical care immediately for the following warning signs. For severe breathing distress, collapse, or inability to wake, call your local emergency number.

Warning signs in adults

Shortness of breath or difficulty breathing.
Persistent chest or abdominal pain or pressure.
Confusion, ongoing dizziness, or difficulty waking.
Seizures.
Not urinating.
Severe muscle pain, weakness, or unsteadiness.
Fever or cough that improves and then returns or worsens.
Worsening of a chronic medical condition.

Warning signs in children

Rapid or difficult breathing, or ribs pulling inward with breaths.
Bluish lips or face.
Chest pain or muscle pain severe enough to refuse walking.
Dehydration, including no urine for eight hours, dry mouth, or no tears.
Reduced alertness or interaction while awake.
Seizures.
Fever above 104°F (40°C) that medicine does not control.
Any fever in a baby younger than 12 weeks.
Returning or worsening fever or cough, or worsening chronic illness.

These lists are not exhaustive. Any severe or concerning symptom deserves medical attention. Source: CDC emergency warning signs.

Who is at higher risk of flu complications?

Higher-risk groups include older adults, young children, pregnant people, and people with certain chronic conditions or weakened immune systems.

Adults aged 65 or older.
Children younger than five, particularly those younger than two.
People who are pregnant or within two weeks after pregnancy ends.
People with asthma, chronic lung disease, heart disease, or diabetes.
People with certain kidney, liver, blood, or neurological conditions.
People with weakened immune systems or a body mass index of 40 or higher.
Residents of nursing homes and other long-term care facilities.

This is a summary, not a complete list. A higher risk does not mean complications are inevitable, and serious illness can also affect people outside these groups. Ask your clinician how your health history affects treatment decisions. Source: CDC higher-risk groups.

How is the flu treated?

Many people with mild flu recover at home with rest, fluids, and suitable symptom relief. Prescription antiviral medicines may be recommended, particularly for people at higher risk or with severe illness.

What can help at home?

Rest, sleep, and drinking enough fluids can support recovery. Fever and pain medicines may help with discomfort when suitable for your age and health. Check labels and ask a pharmacist if you are unsure. Avoid taking multiple products containing the same active ingredient. Source: NHS self-care guidance.

Keep a simple record of the medicines you take and when you take them. This is especially useful if someone else is helping care for you or your child. Do not give aspirin or salicylate-containing products to children or teenagers with suspected flu unless a clinician specifically directs you. Source: CDC care guidance.

Can home remedies cure the flu?

Home care can ease discomfort, but it does not replace medical assessment or prescribed treatment when needed. Rest and fluids are supportive measures. Do not assume a tea, herb, or supplement can cure influenza or prevent complications. If you are at higher risk or becoming more unwell, seek professional advice instead of relying on a home remedy. Source: WHO treatment guidance.

When do antivirals help?

Flu antivirals work best when started within two days of symptom onset. They may still help when started later, especially in people with higher-risk conditions or severe illness, including hospitalized patients.

Do not assume that passing the 48-hour point makes treatment pointless. A clinician can decide whether an antiviral is appropriate. Antibiotics do not treat the influenza virus, although they may be prescribed for a separate bacterial complication. Source: CDC flu treatment guidance.

How long are you contagious with the flu?

People with flu are generally most contagious during the first three days of illness. Some adults can spread it from about a day before symptoms until five to seven days after becoming sick.

Children and people with weakened immune systems may remain contagious longer. Feeling better does not necessarily mean there is no risk of passing the infection to someone else. Source: CDC, How Flu Spreads.

When can you return to work or school?

CDC community guidance advises staying home until, for at least 24 hours, symptoms are improving overall and you have no fever without taking fever-reducing medicine. For the next five days, use added precautions such as cleaner air, good hygiene, a well-fitting mask, or distancing.

If you have suspected or confirmed flu without any fever, CDC flu-specific guidance also advises staying home from work for at least five days after symptoms begin. If fever returns or you begin feeling worse, stay home again. Follow applicable local rules and workplace or school policies. Healthcare settings have separate guidance. Source: CDC respiratory virus guidance.

How can you reduce your risk of flu?

Annual vaccination is the main way to reduce the risk of seasonal flu and serious complications. CDC recommends a yearly flu vaccine for everyone aged six months or older, with rare exceptions.

Vaccination does not prevent every infection, but it can reduce illness severity in people who still get flu. Ask a healthcare professional which vaccine is suitable for you and follow local vaccination guidance.

Other helpful steps include washing your hands, covering coughs and sneezes, avoiding close contact with sick people, and improving indoor ventilation. A well-fitting mask can provide an additional layer of protection. If you are sick, limit contact with others. Source: CDC seasonal flu prevention.

Flu symptoms in 2026: What should you know?

The year alone does not identify your infection or establish a different symptom pattern. Use the symptom guidance above, seek advice based on your health risks, and follow current local public health information.

The CDC's September 2026 prevention guidance continues to emphasize annual vaccination and early assessment for antiviral treatment in people at higher risk. This article does not claim that every circulating flu virus has a unique set of symptoms. Source: CDC seasonal flu prevention guidance.

Frequently asked questions about flu symptoms

Does flu always start with a high fever?

No. Symptoms can begin in different combinations, and some people never develop a fever. Do not use fever alone to decide whether you could have influenza. CDC clinical guidance.

Can the flu cause vomiting or diarrhea?

Yes. Digestive symptoms can occur with influenza, particularly in children. They may accompany respiratory symptoms. CDC clinical guidance.

Can you get flu after having a flu shot?

Yes. Vaccination does not provide complete protection against infection, but it can reduce the risk of flu and the severity of illness. CDC prevention guidance.

Does a negative rapid flu test mean you do not have flu?

No. Rapid antigen tests can produce false negative results. A clinician may consider your symptoms, other information, or additional testing when interpreting the result. CDC testing guidance.

Can influenza A and influenza B cause similar symptoms?

Yes. Both can cause seasonal flu. Your symptom pattern cannot reliably identify the virus type, so do not assume that one particular symptom means influenza A or B. WHO seasonal influenza guidance.

Should you wait for a positive test before calling a doctor?

No. Contact a healthcare professional promptly if you are at higher risk or feel very unwell. Seek immediate care for emergency warning signs, regardless of test availability. CDC care guidance.

Sources and further reading

This article provides general health information and does not replace an individual medical assessment.