It has already been responsible for around 300 deaths.
The potentially deadly Australian flu has spread to the UK, with health bodies reporting a large rise in cases.
In Ireland, the Irish Health Service Executive says a number of people have died from Australian flu.
And Public Health England says a growing numbers of cases have been reported in the past two weeks by GPs and hospitals.
Among those hit by the outbreak is millionaire sportsman Conor McGregor. The 29-year-old UFC fighter said he had been left “shaking in bed the past two days” by the nasty illness.
The strain, known as H3N2, has already caused hundreds of deaths Down Under and has affected up to 170,000 people. People are being urged to get a flu jab to protect themselves.
The flu is a subtype of influenza A and mainly affects pregnant women, the elderly and children.
Those with health conditions are also at risk of the bug which can lead to pneumonia and other health complications.
What are the symptoms of Australian flu?
The symptoms of most flu, including H3N2, are similar, but different strains can be more severe or contagious than others.
The NHS says flu symptoms come on very quickly and can include:
- a sudden fever – a temperature of 38C or above
- aching body
- feeling tired or exhausted
- dry, chesty cough
- sore throat
- difficulty sleeping
- loss of appetite
- diarrhoea or tummy pain
- nausea and being sick
The symptoms are similar for children, but they can also get pain in their ear and appear less active.
How to treat flu yourself if you have flu
To help you get better more quickly, the NHS advises you to:
- rest and sleep
- keep warm
- take paracetamol or ibuprofen to lower your temperature and treat aches and pains
- drink plenty of water to avoid dehydration (your pee should be light yellow or clear)
A pharmacist can give treatment advice and recommend flu remedies.
Be careful not to use them if you’re taking paracetamol and ibuprofen tablets as it’s easy to take more than the recommended dose.
Speak to a pharmacist before giving medicines to children.
When should you see your doctor if you have flu?
You should see a GP if:
- your symptoms don’t improve after seven days
- you’re worried about your child’s symptoms
- you’re 65 or over
- you’re pregnant
- you have a long-term medical condition – for example, diabetes or a heart, lung, kidney or neurological disease
- you have a weakened immune system – for example, because of chemotherapy or HIV