Influenza (seasonal) - including symptoms, treatment and prevention

See the Stop the Flu Before It Stops You page for information about flu vaccination.

Influenza, commonly known as the flu, is a highly infectious viral illness caused by influenza A or B viruses. It affects the nose and throat and may also affect the lungs.

In Australia, seasonal flu of varying severity occurs every year, usually between May and September. 

Influenza is a notifiable condition¹

How influenza is spread

The flu virus is spread when an infected person talks, coughs or sneezes small droplets that contain the virus into the air where they may be breathed in by people nearby. Infection may also be spread by contact with hands, tissues and other infected articles.

Signs and symptoms

Symptoms include:

• rapid onset of fever
• headache
• muscle aches
• fatigue
• sneezing
• runny nose
• sore throat
• cough
• nausea, vomiting and diarrhoea in children.

Most people recover within a week, although the cough and fatigue may last longer. Flu is much more serious than the common cold. It can lead to pneumonia (lung infection or inflammation) and other complications, and even death, particularly in:

• people aged 65 years and over
• pregnant people
• young children 
• people with chronic conditions like heart disease, diabetes and lung disease.

Influenza and pregnancy

Pregnant women are more likely to have severe complications from the flu, especially in the second and third trimesters. While the flu virus does not cross the placenta and infect the baby while in the uterus, high fever and any chest complications caused by flu can be potentially harmful to the baby.

To lower risk, a doctor may recommend antiviral medication for pregnant women with the flu.

There is no evidence that antiviral medications used in Australia to treat flu are associated with adverse effects in pregnancy. Similarly, women may continue to breastfeed while taking antiviral medication.

The best protection for the mother and baby is for the pregnant mother to have the flu vaccine, which is safe and funded at any stage of pregnancy.

Diagnosis

Not all people with symptoms need to be tested for flu. The diagnosis may be suspected on clinical symptoms and examination especially during the flu season. Diagnosis may be confirmed by laboratory testing of mucus from the back of the nose or throat.

People in high-risk groups should speak to their GP about flu testing.

High risk groups include:

• children aged 5 years and younger
• people aged 65 years or older
• a resident of an aged care or long term residential care facility
• Aboriginal and Torres Strait Islander people
• pregnant women
• people with long-term medical conditions
• people with a disability with significant or complex health needs or multiple comorbidities
• people who are obese (BMI 30 or more)
• people experiencing homelessness.

Combined COVID-19 and flu testing is available for high risk groups by GP referral only.

Incubation period

(time between becoming infected and developing symptoms)

Incubation is one to four days (average of two days).  

Infectious period

(time during which an infected person can infect others)

Usually from one day before onset of symptoms until seven days after the onset of symptoms.

After five days the level of infectiousness is probably very low, however some people, especially children and people with weakened immune systems, might be able to infect others for a longer time.

Treatment

Most people recover with rest, drinking plenty of fluids and use of paracetamol for the relief of pain and fever. Aspirin should not be given to children under 12 years of age unless specifically recommended by a doctor.

People with moderate or severe illness, pregnant women, Aboriginal and Torres Strait Islander people and those with chronic medical conditions may benefit from specific antiviral medication. This can reduce duration of symptoms by about 1 day and prevent some of the more serious complications of flu but is only effective if commenced within 48 hours of illness onset.

Antiviral therapy may sometimes be used to prevent infection in close contacts of people with flu, such as vulnerable household contacts. A contact is any person who has been close enough to an infected person to be at risk of infection themselves.

When to seek medical advice

Seek medical advice if you experience:

• shortness of breath or difficulty breathing
• confusion
• worsening or concerning symptoms.

Seek medical advice for a child if they:

• are unable to keep fluids down because of vomiting
• children show signs of dehydration (dizzy on standing, passing less urine than usual).

Prevention

• Flu vaccines reduce the risk of getting severe influenza. Annual flu vaccination is strongly recommended for anyone 6 months of age and older, especially people at increased risk of complications. 
• Cover a cough or sneeze with a tissue or your arm, not with your hand. Drop used tissues immediately into a rubbish bin, then wash your hands.
• Wash hands as soon as possible after sneezing, coughing and after contact with items contaminated with nose and throat discharges. Use soap and water or an alcohol-based hand rub.
• Wipe down all frequently touched surfaces regularly with a cleaning cloth dampened with detergent, or a large alcohol wipe.
• Exclude people with flu from childcare, preschool, school and work.

1 - In South Australia, the law requires doctors and laboratories to report some infections or diseases to SA Health. These infections or diseases are commonly referred to as 'notifiable conditions'.