H What N What?

In case I haven't been clear about this, the swine flu outbreak really has me nervous. Not in a stockpile-food-and-ammo way...but it's definitely something that I'm keeping a close eye on. There are a few constants throughout all of human history. War is one of them. Plague is another.

So I thought I'd spend a little time here talking about what the flu is, beginning with how flu viruses are named.


Viruses are, essentially, nucleic acid particles surrounded by a protein capsid and a lipid envelope. The nucleic acid can be either DNA or RNA, and in the case of influenza its the latter. Sticking off of the outer coating are little markers called "antigens". While there are many different types of antigens, two in particular are important: one called hemagglutinin and another called neuraminidase. For influenza viruses, there are 16 known types of hemagglutinin antigens and 9 known types of neuraminidase antigens. Since a particular virus strain will only have one of each, they are referred to by the types of these two antigens present.

For example, the current swine flu that is threatening to become a global pandemic is designated H1N1. This means that it has the type 1 hemagglutinin antigen and the type 1 neuraminidase antigen. The bird flu outbreak a few years ago in souteast Asia was type H5N1. In the past 100 years, there have been pandemic outbreaks of both H2N2 and H3N2, along with the grandaddy of all flu pandemics, the 1918 influenza outbreak which was (like this year's threat) an H1N1 virus.

The problem with the flu virus is that it frequently mutates, and any mutations typically affect the antigens on the outer surface of the virus. It's these antigens that the body's immune system recognizes and attacks. So by constantly changing these to new forms, viruses are able to easily evade the body's defense mechanisms. This is one reason why it's necessary to get a new flu shot every year (and why many people each year catch the flu even after getting vaccinated against it)...the antigens on the surface of a viral strain will change enough from year to year that the body won't recognize it.*

*In contrast, antigens on the surface of the virus that causes measles do not mutate, which is why the measles vaccination that most of us got as children should provide lifetime immunity.

Strains like this H1N1 "swine flu" that has come out of Mexico are especially scary because they are mutated forms of an animal virus. In the case of this one, it's a hybrid of pig, bird, and human viruses. What this means is that, while the combination of antigen types (H1N1) is nothing new to humanity, our species has never before been exposed to the actual antigens of this virus. There is, therefor, very little if any natural immunity to it. There is, therefor, a much greater likelihood of it becoming a global killer.