Flu Symptoms: What to Expect and When to See a Doctor

The flu (influenza) is a highly contagious viral infection that affects the whole body, including the nose, throat, and lungs. Flu symptoms can range from mild to severe and may vary depending on:

• whether you are experiencing influenza A symptoms or influenza B symptoms

• age (flu symptoms in kids can be different to flu symptoms in adults)

• whether you have early flu symptoms or flu symptoms later in the course of illness . 

• the presence of pre-existing health conditions

• complications developing from influenza

• If you have been vaccinated against the flu

• If you have had an influenza infection before

This guide explains what to expect from early signs like fatigue and sore throat to more serious symptoms such as chest pain or shortness of breath. We also cover how symptoms progress, how flu compares to similar illnesses such as the common cold, and when it’s time to see a doctor.

What Are the Symptoms of the Flu?

Flu symptoms usually come on suddenly, and commonly include any of the following:

• Fever (usually 37.8 to 40℃, but can be higher¹

• Cough (usually dry)

• Body aches

• Headache

• Blocked or runny nose

• Malaise

• Sore throat

• Nausea

Other flu symptoms that can occur include:²

• Dizziness

• Weakness

• Reduced appetite

• Abdominal pain

• Diarrhoea or vomiting

• Red eyes or eye irritation

If you notice flu symptoms, especially during the flu season (generally June to September in Australia), it is important to make an appointment with your GP as soon as possible to see what treatment is right for you. In some people, antiviral medications can help reduce the chance of flu symptoms getting worse, and work best if started as early as possible.

How symptoms of influenza come on can vary with the type of influenza. However, it is not possible to tell which type of influenza is causing flu symptoms without testing. Your GP can help you decide whether testing for influenza is right for you.

Influenza A Symptoms

Influenza A is more likely to cause sudden and intense flu symptoms. Influenza A symptoms usually come on quickly and feel more severe than a common cold. Importantly, people with influenza A symptoms are more likely to be infectious in the first 1-3 days of flu symptoms.²

Read more in our Influenza A Overview.

Influenza B Symptoms

Influenza B symptoms can be similar to influenza A symptoms. Influenza B can affect people of any age, but has been associated with spread among school aged children.³

Studies in children suggest that symptoms of Influenza B are also more likely to include the following:⁴

• Calf pain

• Diarrhoea

Unlike Influenza A, Influenza B is most infectious before symptoms start (around 48 hours), as well as 1-2 days after symptoms start.⁵

See our full Influenza B Overview.

Early Flu Symptoms

Because flu symptoms often come on suddenly, they are usually the strongest early in the course of illness (ie. the first 1-3 days). Getting plenty of rest and staying hydrated during this stage can help reduce how long the flu lasts and how severe it becomes. It’s important to watch for symptoms that worsen, such as persisting  fever, difficulty breathing, or chest pain. People are generally most infectious at this time so it is important to avoid spreading the flu to others who are more likely to have complications from the flu, such as older adults (>65 years old) and children. Making an appointment with your GP as soon as possible is important, as antiviral medications may help and work best if started early in the illness.

Flu Symptoms in Adults vs Children

In Older Adults

Adults over the age of 65 with Influenza are more at risk of complications.⁶ In addition, flu symptoms can differ compared with younger adults, such as:^2,7

• Dizziness

• Malaise and weakness

• Reduced appetite

• Drowsiness or confusion

The typical flu symptoms of fever, sore throat or muscle aches can even be absent in older adults with Influenza infection. 

If you are or someone you care for is an older adult with suspected flu symptoms, make an appointment with your GP as soon as possible to discuss the best treatment.

In Children

Children often show similar flu symptoms as adults, but are more likely to haveother symptoms such as:

• Swollen neck lymph nodes (cervical lymphadenopathy)

• Abdominal pain

• Vomiting and diarrhoea⁸

Flu symptoms can develop rapidly in kids and sometimes lead to complications like ear infections or dehydration. Children under the age of 5 years old are at highest risk of complications of the flu and needing hospital, and infants under 6 months old are at particular risk.2,9 If you suspect your child has influenza, it is important to make an appointment with your GP or seek medical attention as soon as possible to ensure they receive the best treatment.

How Long Do Flu Symptoms Last?

Average Duration

Flu symptoms usually last between 3 to 7 days in most people.¹⁰ However, coughing and feeling tired or weak can continue for several days or even weeks after other symptoms have eased.²

When Symptoms Linger

Flu symptoms (such as fever) should generally be improving after 3-7 days, with most people recovering within a week. It’s common for some symptoms, especially tiredness and cough, to last 1 to 2 weeks after the main flu symptoms improveOlder adults and people with chronic lung disease are more likely to have persisting malaise and cough.² If symptoms persist beyond 7 days or worsen, it’s important to see a GP. This helps rule out complications like secondary infections or other illnesses. Early medical advice can support a safer recovery, particularly for young children, older adults or those with existing health conditions.

Flu Complications and Warning Signs

Common Complications

Flu symptoms that continue or come back beyond 3-5 days of starting makes complications more likely. There are many different complications that can occur, ranging from mild to severe needing emergency care. Some examples of complications of the flu are:²

• Pneumonia

• Shortness of breath

• Fast breathing

• Persisting fever

• Heart complications

• Chest discomfort

• Palpitations or dizziness

• Fainting

• Complications in young children

• Middle ear infections

• Croup

• Bronchiolitis

• Myositis

• Worsening of pre-existing conditions

• Asthma

• Heart disease

• Diabetes

Early discussion with your GP is important to ensure your immunisations are up to date, as this reduces risk of complications.  

When Symptoms Worsen

Watch for warning signs like trouble breathing, chest pain, feeling dizzy or confused, a very high fever that won’t go down, or a cough that gets worse after seeming to improve. These symptoms need immediate medical attention. If you notice any of these, call your GP or go to the emergency department without delay. Acting quickly can help prevent serious complications and protect your health. 

Don’t hesitate to get help. Your safety is important.

Is It Flu, Cold, or COVID?

Symptom Comparison Table

What to Look For

Flu usually starts suddenly with fever, aches, and tiredness. A cold develops slowly, mainly causing a runny or stuffy nose and mild sore throat. COVID-19 symptoms can vary but often include loss of taste or smell, which is less common in flu and colds. Because these illnesses share many symptoms, it can be tricky to tell them apart without testing. If you’re unsure or symptoms get worse, it’s best to see your doctor or get tested. 

Early diagnosis helps you get the right care and protect others.

When to Speak to a Doctor

If your symptoms get worse, your fever lasts longer than a few days, or you’re in a high-risk group for influenza complications (young children, older adults, pregnant people, or those with chronic health issues), it’s important to see a GP without delay. Look out for serious signs such as difficulty breathing, chest pain, confusion, or signs of dehydration as these need urgent care.