Health personnel hold a container with H7N9 viral specimens from China at the National Influenza Centre in Taiwan.
Flu comes in many forms. In China right now they're battling human infections of H7N9, in 2009 it was H1N1 and in 2004-5 it was H5N1.
Why all the code names, and what do they mean?
Viruses, including influenza viruses, are machines. Anyone who doubts this hasn't seen a bacteriophage virus up close.
Influenza viruses look more like the Death Star: roughly spherical and covered with spikes.
Inside is just enough genetic material and protein to ensure the virus can take over host cells and replicate itself, but it's actually the spikes on the outside that inspire the code names.
The spikes are made of two different proteins: haemagglutinin, or HA, and neuraminidase, or NA. The HA is what attaches to the wall of a host cell and tricks the cell into absorbing the virus.
The NA does its work at the other end of the virus life-cycle, detaching newly created viruses from the cell so they can head off to spread the infection.
Of the two main types of influenza that affect humans (known as A and B), influenza A viruses have the wider genetic diversity because they also infect many other species. This diversity comes out in the different kinds of HA and NA that viruses have on their surface, with more than a dozen different subtypes of HA known and almost as many kinds of NA. To introduce some order each protein subtype is given a number and the combination of these numbers is used to categorise each kind of influenza virus: a virus with HA subtype 1 and NA subtype 1 is called H1N1 for short.
Influenza B viruses are similar to those of influenza A, but because they infect a much narrower range of hosts (just us and seals) they also have a narrower range of HA and NA subtypes so the code names aren't used.
Even within a single subtype of HA or NA there is considerable variation as the viruses evolve, which is why it's impossible to develop permanent immunity to the flu. The body fights these viruses with antibodies that latch on to HA and NA proteins and deactivate them. But antibodies that target HA subtype 1 this year won't necessarily work next year once the shape of the HA molecules has evolved slightly.
And if an unfamiliar subtype of influenza A jumps to humans from another species our immune systems can be left completely unprepared, a situation which can lead to a pandemic.
Different types of influenza A tend to be associated with different kinds of animals, which is where labels such as "bird flu" for H5N1 and "swine flu" for H1N1 originate.
Ultimately, it is thought, aquatic birds are the source of influenza A and act as symptomless carriers of most types (insert your own joke about quack medicine).
Correct virus classification is crucial for creating seasonal vaccines and catching potential pandemics as early as possible – though when you're stuck in bed with the flu it somehow seems less crucial to know if it's H1N1 or H3N2.