What does "swine flu" and "H1N1" actually mean?

I'm always embarrassed when my education fails me.  Last night while at dinner with friends, I was asked what "H1N1" meant.  I quickly responded with a guess that it was the chemical composition of the core antigenic part of the virus.  One hydrogen, one nitrogen.  Not only was that wrong, but it doesn't even make much sense from a chemical standpoint.  So, I'm here to set the record straight.

What is "H1N1"

Aside from the "butt" of a fantastic joke, it is a serologic classification which identifies the exact subtype of the influenza virus. You see, there are two main proteins on the envelope of the influenza virus—hemagglutinin (H!) and neuraminidase (N!)—that are critical to the mechanism of infection and replication of the influenza virus.  These two proteins are known to exist in a number of different configurations, 16 for hemagglutinin and 9 for neuraminidase[1]  and each configuration is numbered. This means that H1N1 is hemagglutinin type 1 and neuraminidase type 1. H1N1 has the same hemagglutinin protein structure as H1N2 and the same neuraminidase protein structure as H5N1.

Why is this important?

This stuff isn't just taxonomic fluffery.   For those who don't know or remember, the proteins which exist on the surface of a pathogen and the structure of those proteins is like a key which fit very particular locks (receptors) on the surface of the certain of the body's cells creating some change in the cell.  Changes in these configurations can completely alter the way that the pathogen works.  Because these proteins are so unique, they are also markers that the body remembers and recognizes in its immune response and hence are critical to the creation of vaccines against the pathogens.

In short, these proteins are the critical actors in both the mechanisms of infection and the identifiers in the war against them.

Particular to the influenza virus and our two proteins, Wikipedia tells us that Hemagglutinin "mediates binding of the virus to target cells and entry of the viral genome into the target cell" and neuraminidase "is involved in the release of progeny virus from infected cells".  To use a metaphor, if flu virus infection was a bank robbery, hemagglutinin would be the explosives expert responsible for blowing a hole in the wall of the bank and neuraminidase would be the safe cracker and getaway man.

Different configurations of these proteins present differently-abled virus subtypes.  One configuration may be more resistant to conditions outside of the body, another might be more easily communicable from host to host, others might have more devastating effects on the infected host. So knowing what virus we're dealing with not only helps us develop a vaccine, but also informs us of its typical characteristics.

What is swine flu?

You may have noticed recently that the World Health Organization and other related agencies just stopped calling the virus the "swine flu" and started calling it by its scientific name Influenza type A subtype H1N1.  Some falsely believe that this was a worldwide response to cries of foul from pig farmers worried about their business, to stem the worldwide panic causing countries to slaughter pigs and the general public to have sworn off bacon and other lesser pork products.  While those factors all certainly exist, in fact, this was the correct thing to do epidemiologically speaking.

"Swine flu" is just a generic term for any influenza virus that is transmitted between pigs.  Once the flu crosses over to another species, like us, it's no longer a "swine flu" but a "human/dog/seal/bird flu of swine origin".  So calling the current Influenza A H1N1 virus a swine flu doesn't cirrectly communicate the human-to-human transmissibility aspect of the virus.  H1N1 has, just like in 1918, become a "human flu".

Related Articles

• jay.vox.com: Why is swine flu more dangerous than normal flu?
  
• Fluidinfo.com: A few comments on pandemic influenza
• Fluidinfo.com: OK, it’s a pandemic. Now what?
• virology.ws: Influenza Virus Transmission
• virology.ws: Structure of the Influenza Virus
• Wikipedia: Swine Flu
• Wikipedia: Influenza
• Wikipedia: Influenzavirus A
• Wikipedia: Influenza A virus subtype H1N1
• Wikipedia: 2009 Swine Flu Outbreak