The British media have blamed "Aussie flu" for the outbreak. The truth is, there is not just one flu strain we should be worried about, and "Aussie flu" is a bit of a misnomer.
First, a bit about flu strains. There isn't really a flu virus. Flu virus is a name we give a group of four closely related viruses: influenza A, influenza B, influenza C and influenza D.
While humans can't catch influenza D (that's for pigs and cows), we can be infected with influenza A, B and C. Public health officials, however, are less worried about influenza C as it isn't a major cause of illness. But influenza A and B are a real worry.
Influenza A has been found in – and causes disease in – lots of animals, including birds, bats, dogs, pigs and penguins.
One of the major worries is pandemic influenza, where a new virus jumps from animals and spreads across the world easily because we haven't had a chance to build up immunity to that new type.
Influenzas A and B can be subdivided even further by the proteins they carry on their surface – hemagglutinin (H) and neuraminidase (N). These proteins help the virus identify the right cells to infect.
For influenza A there are 18 Hs and 11 Ns identified so far. Hence we get names such as H1N1 for swine flu or H5N1 for bird flu.
Contrast this with the fact that there are really only two lineages of influenza B, named after cities in Japan and Australia: Yamagata and Victoria, respectively.
The Hs and Ns are continuously evolving in response to our immune systems, which recognise and make antibodies to stop the virus taking hold.
A vaccine usually supplies the H and N proteins without the potentially dangerous virus. Scientists also continuously track the H and N of circulating influenza viruses and adjust the vaccine to match what's out there.
This is the basis for flu vaccination and why you have to get a new vaccine jab every year.
Enter 'Aussie flu'
"Aussie flu" refers to one kind of influenza A virus strain, the H3N2 strain.
The southern hemisphere, including Australia, just experienced one of its worst influenza seasons in recent history and this is the virus that has reached British shores.
But we don't actually know where the virus originated from. All we can say is, it probably wasn't from Australia.
One place it is more likely to have come from is the sub-tropical regions that do not have winter seasons.
These regions do not suffer from the same large flu epidemics that temperate countries like the UK and Australia have (we don't know why, but some scientists have suggested it's to do with temperature or humidity), but have continuous lower-level circulation of flu that allows influenza viruses to persist between winters.
What's worrying about this season is the experience Australia had last flu season. Australia was hit particularly hard by influenza virus H3N2. H3N2 is a typical seasonal flu strain – like H1N1 – but it tends to be more difficult to control.
There are three red flags this flu season, and they are that H3N2 viruses typically causes more hospitalisations and deaths in older people, there are difficulties in producing effective H3N2 vaccines (explained below), and there's more than just H3N2 to consider, especially in the UK this year.
Although good against the other strains, this season the vaccine is about 20 percent protective against H3N2 viruses (not great, but better than nothing) as the virus changed unavoidably during production.
This is due to a quirk of how flu vaccines are produced. They are grown in chicken eggs, and then inactivated before being used in vaccines.
Flu viruses mutate quickly and they mutate to adapt to their environment. Of course, a chicken egg is a different environment to a human body, so the end result may be a virus that's not best suited to a flu vaccine. This appears to have been what happened with the latest H3N2 vaccine.
Seasonal flu epidemics are usually caused by a mixed bag of viruses. This year, the mix is so far mainly shared between H3N2 and influenza B.
What's worse is that this increase in proportion of influenza B makes it more difficult to protect from because the most popular vaccine in the UK is a "trivalent" that protects against three flu viruses (H1N1, H3N2 and one of the two kinds of influenza B).
This year, though, the other type of influenza B (Yamagata) is more common meaning that those with the trivalent vaccine will be protected less, although they would likely get some cross-influenza B protection.
One vaccine to rule them all
Influenza is incredibly diverse. And this diversity can have devastating consequences for human and animal health. Although our ability to track flu, predict the viruses making up the next season and produce safe and effective vaccines is improving, we are always playing catch up.
Efforts to produce a universal flu vaccine are, however, being pursued by scientists across the world. The idea would be that a single vaccine given a few times during your life would protect you from any flu virus, irrespective of H, N, A or B.
But, until then, you can defend yourself and your loved ones from the flu this year by getting your vaccine, practising good personal hygiene, such as handwashing, and avoiding crowded spaces if you are experiencing flu-like symptoms.