Influenza (Flu) Symptoms: Early Signs, Treatment and When to See a Doctor
Disclaimer: The content provided is for general informational purposes only and does not constitute medical advice, nor is it a substitute for professional medical consultation. Always seek the guidance of your doctor or other qualified health professional with any questions you may have regarding your health or a medical condition. For emergencies please immediately contact 000. Any medical topics discussed are intended to educate only.
Influenza, commonly known as the flu, can leave you feeling well one day and significantly unwell the next.
Unlike the common cold, which often develops gradually, flu symptoms can appear suddenly. Fever, chills, body aches, headache, fatigue and cough can develop within hours and may be enough to keep you away from work, study and your usual activities.
Recognising the early symptoms of influenza can help you take appropriate steps sooner, reduce the risk of spreading the infection to others and know when it may be time to seek medical advice.
In some circumstances, antiviral treatment may also be considered, particularly for people at increased risk of complications. These medicines work best when started early in the illness, generally within the first 48 hours of symptoms.
If you are concerned about flu symptoms, an online GP consultation can be a convenient starting point. However, some symptoms require an in-person examination or urgent medical care, and telehealth is not appropriate for medical emergencies.
What is influenza?
Influenza is a contagious viral infection caused by influenza viruses. It primarily affects the respiratory system, including the nose, throat and lungs.
Influenza spreads mainly between people through respiratory particles released when an infected person breathes, talks, coughs or sneezes, particularly during close contact.
Flu is different from the common cold. While both can cause respiratory symptoms, influenza is more likely to cause a sudden onset of fever, significant fatigue, headache and muscle or body aches.
In Australia, influenza infections are most common during the winter months, particularly from around June to September, although infections can occur at other times of the year.
What are the symptoms of influenza (flu)?
Common influenza flu virus symptoms include:
- Fever or feeling feverish
- Chills or shivering
- Cough
- Sore throat
- Runny or blocked nose
- Headache
- Muscle aches and joint pains
- Significant tiredness or fatigue
- Reduced appetite
Some people may also experience nausea, vomiting or diarrhoea. Gastrointestinal symptoms can occur with influenza but are more commonly seen in children.
Not everyone with influenza develops every symptom, and it is possible to have influenza without a significant fever.
What are the early signs of influenza?
One of the characteristic features of influenza is how quickly symptoms can develop.
Early flu symptoms may include:
- Suddenly feeling unusually tired or unwell
- Fever or chills
- Headache
- Muscle or body aches
- Sore throat
- Dry cough
Someone may feel relatively normal earlier in the day and become significantly unwell within several hours.
This rapid onset is one of the features that can help distinguish influenza from a typical cold, although symptoms alone cannot confirm which respiratory virus you have.
What is the difference between influenza A and B?
Influenza A and influenza B are the two main types of influenza virus responsible for seasonal flu in humans.
Both can cause very similar symptoms, including fever, cough, sore throat, headache, fatigue and body aches.
Influenza A is frequently associated with seasonal outbreaks and has the potential to cause pandemics.
Influenza B also causes seasonal influenza and commonly affects children as well as adults.
From an individual's perspective, however, you generally cannot tell whether you have influenza A or B from your symptoms alone.
Testing, such as a PCR test or an appropriate rapid antigen test, may be used to confirm influenza and distinguish between influenza A and B when clinically indicated.
Flu vs cold: how can you tell the difference?
Colds and influenza can share several symptoms, but there are some typical differences.
| Symptom | Common cold | Influenza |
|---|---|---|
| Onset | Usually gradual | Often sudden |
| Fever | Less common | Common |
| Body aches | Usually mild | Can be significant |
| Fatigue | Usually mild | Often moderate to severe |
| Headache | Less common | Common |
| Cough | Common | Common and may be prominent |
| Complications | Less common | More likely |
These differences can provide clues, but they cannot diagnose influenza.
COVID-19 and other respiratory viruses can cause many of the same symptoms. Testing may therefore be required to determine which infection is responsible.
How soon do flu symptoms appear after exposure?
The influenza incubation period is usually around 1 to 4 days, with symptoms commonly developing approximately 2 days after exposure.
This means you may not realise immediately that you have been infected.
How long does influenza last?
The exact course varies between individuals.
For many otherwise healthy adults, the most significant flu symptoms improve over several days, with recovery occurring over approximately a week. However, cough and fatigue may persist for longer.
A general pattern may look like:
- Days 1–3: Symptoms can begin abruptly, including fever, chills, headache, sore throat and body aches.
- Days 3–5: Cough, congestion and fatigue may remain prominent.
- Days 5–7: Fever and other acute symptoms often begin improving.
- 1–2 weeks: Many people have recovered, although tiredness or cough can sometimes linger.
This is only a guide. Influenza does not follow exactly the same timeline in everyone.
Older adults, young children, pregnant women and people with certain underlying health conditions may be at greater risk of severe disease or complications.
Is influenza contagious?
Yes. Influenza is highly contagious.
People may be able to transmit influenza before they realise they are unwell and can continue spreading the virus after symptoms begin.
If you develop flu symptoms, staying home while unwell, practising good hand and respiratory hygiene and avoiding close contact with vulnerable people can help reduce transmission.
What should you do when flu symptoms start?
For uncomplicated influenza, treatment is usually focused on relieving symptoms while your immune system clears the infection.
Rest
Allow yourself time to recover. Continuing strenuous activity while significantly unwell can make it harder to manage your symptoms.
Drink plenty of fluids
Fever and reduced fluid intake can contribute to dehydration. Water and other suitable fluids can help maintain hydration.
Manage fever and body aches
Medicines such as paracetamol or ibuprofen may be appropriate for some people to relieve fever, headache and muscle aches. Always follow the directions and check with a doctor or pharmacist if you are unsure whether a medicine is suitable for you.
Reduce the risk of spreading influenza
Stay home while you are unwell and avoid close contact with other people where possible, particularly those at higher risk of complications.
Consider speaking with a doctor early
If you are significantly unwell or at increased risk of complications, getting medical advice early can be important because some influenza treatments are most effective when started soon after symptoms begin.
Are there antiviral medicines for influenza?
Yes. Prescription antiviral medicines can be used to treat influenza in certain circumstances.
Antivirals are not required for everyone with influenza. Many otherwise healthy people with mild influenza recover with rest, fluids and symptomatic treatment.
A doctor may be more likely to consider antiviral treatment if you are at increased risk of severe influenza or complications, are pregnant or recently postpartum, require hospital treatment, or have significant illness.
When indicated, influenza antivirals are most effective when treatment begins within approximately 48 hours of symptom onset.
A doctor can assess your symptoms, medical history and individual risk factors to determine whether antiviral treatment is appropriate. Where clinically suitable, a prescription can be issued through an online prescription service.
Antibiotics do not treat influenza itself because influenza is caused by a virus. They may only be required if a bacterial infection develops as a complication.
Who is at higher risk of complications from influenza?
Although anyone can become seriously unwell with influenza, some people have an increased risk of complications.
This includes:
- Babies and young children
- Older adults
- Pregnant women
- Aboriginal and Torres Strait Islander people
- People with certain chronic medical conditions
- People with conditions or treatments that affect their immune system
People in higher-risk groups should consider seeking medical advice earlier if they develop influenza symptoms.
When should you see a doctor for flu symptoms?
Consider speaking with a doctor if:
- Your symptoms are severe or getting worse
- You are struggling to maintain adequate fluids
- Your fever persists or you remain significantly unwell
- You initially improve and then become unwell again
- You are pregnant
- You are older or medically vulnerable
- You have an underlying medical condition that increases your risk of complications
- You are concerned about your symptoms or recovery
Early assessment can be particularly helpful for people who may benefit from antiviral treatment.
When is influenza an emergency?
Seek urgent medical attention if you or someone you are caring for develops symptoms such as:
- Significant difficulty breathing
- Severe or persistent chest pain
- Blue or grey discolouration of the lips or face
- New confusion, collapse or loss of consciousness
- Severe dehydration
- Extreme weakness or difficulty staying awake
- Persistent vomiting with an inability to keep fluids down
- Rapid deterioration or other severe symptoms
Call 000 in a medical emergency. Telehealth should not be used as a substitute for emergency medical care.
How is influenza diagnosed?
During influenza season, a doctor may be able to make a clinical assessment based on your symptoms and circumstances.
When confirmation is required, influenza can be detected using a PCR test or an appropriate rapid antigen test (RAT). Some combination RATs can test for influenza A, influenza B, COVID-19 and sometimes RSV.
Testing may be particularly useful when the result could influence treatment or when distinguishing influenza from another respiratory infection is important. Where appropriate, a doctor can arrange a pathology referral for further assessment.
Can you prevent influenza?
The most effective way to reduce your risk of influenza and serious influenza-related illness is annual influenza vaccination.
In Australia, annual influenza vaccination is recommended for everyone aged 6 months and over. Some groups are eligible for free vaccination under the National Immunisation Program.
Because circulating influenza strains can change over time, influenza vaccines are updated and vaccination is recommended each year.
Other practical measures include good hand hygiene, covering coughs and sneezes, improving ventilation where possible, and avoiding close contact with others when you are unwell.
Can an online doctor help with influenza?
For appropriate non-emergency symptoms, telehealth can provide a convenient way to speak with an Australian-registered doctor without needing to leave home while you are unwell.
Depending on your clinical circumstances, a MediLeave doctor may be able to provide:
- Assessment of flu and respiratory symptoms
- Advice about symptom management
- Advice about influenza testing
- Prescription treatment when clinically appropriate
- An Online Medical Certificate when clinically appropriate
- Advice about influenza vaccination
- Referral for pathology or further assessment where appropriate
- Referral to in-person or emergency care when required
Every MediLeave doctor is AHPRA-registered.
Telehealth does have limitations. Some respiratory symptoms require physical examination, measurement of vital signs, pathology testing or urgent hospital assessment. Your doctor will advise you if face-to-face care is more appropriate.
If you are experiencing a medical emergency, call 000.
The takeaway
Influenza can come on quickly and cause much more significant symptoms than the common cold. Sudden fever, chills, body aches, headache, fatigue and cough are among the most common signs.
Most otherwise healthy people recover with rest, adequate fluids and appropriate symptom management. However, early medical advice can be important if you are at increased risk of complications or have significant symptoms, particularly because influenza antiviral medicines are most effective when started early.
If you're experiencing flu-like symptoms and would like medical advice from home, MediLeave can connect you with an Australian-registered doctor for an online consultation, with referral to in-person care where clinically required.
