Naming the flu: H-something, N-something

So we often talk about influenza in terms of letters. I'm sure you've heard of H1N1, or something like that. And what I wanted to do is explain where that naming system comes from, what it means, and how it relates to the three types of influenza that exist--A, B, and C. Now let's say I went around the world collecting all the different influenzas I could find. I might find some type C influenzas. I might find some type B influenzas, as well. And I would find probably lots and lots of type A's, because there is so much diversity among the A's. Lots of different types. Now, when you have a few viruses, it's fine to say, well, it's a type B, or it's a type C. But when you have this many viruses like I'm drawing for type A, it becomes a little overwhelming to simply say, well, it's a type A. Because you're probably thinking, well, there are so many different types, tell me more. I want to know more about it. And so in order to find out more about it, or tell someone more about it, what people have come up with is a new naming system. Or kind of a different naming system, other than just the letter. So what they do is they say, OK, let's go back to what the virus looks like. We know that the virus has kind of an outside, an envelope, right? That's what I'm drawing here. And on the inside, the influenza virus has RNA. And if it's a type A virus or a type B virus, that means it has eight chunks of RNA. The type C actually has only seven. So this one that I drew here, this must be a type A or B. And I'm going to go ahead and tell you it's a type A. And we know that on the outside type A's have some proteins, right? They have some proteins I'm drawing as kind of a little hand, because this is to remind me that it's an H protein, or hemagglutinin protein. And they also have a type N protein. And so type H and type N. Remember, H is hemagglutinin, helps the virus get into cells, or hold onto sialic acid. And type N is neuraminidase, and it kind of nicks the sialic acid, and helps the virus get out of a cell. Now, scientists have been looking at these proteins for awhile, and they've actually counted all the different types of H proteins they could find. They found that there are about 17 types, meaning they all do the same thing, they all have the same job, but they're slightly different. Their proteins and their structure might be slightly different. And if you count up all the different N proteins, there are actually only 10. So 10 different types of N proteins, and 17 types of H proteins. Now, if every virus has to have some H and some N, how many different combinations would you get? Well, you simply multiply them, right? You simply multiply 17 times 10, and that means there are 170 combinations you can actually come up with in terms of different types of H's and N's coming together. So if that's the total number, then we can use that to actually name the different types of influenza A's. You can actually say, well, maybe this guy has the third type of H and the second type of N. And if that was the case, we'd call it an H3N2. And maybe this guy has the first type of H, and the first type of N, and that would be H1N1. And maybe this guy is H5N1. Or maybe this is H7N2. I think you get the idea. Basically, you just kind of name then based on the H and the N that they have on the outside. Now here's something to think about. What happens if you actually look at this little virus, and you find that this virus has the first H and the first N. Does that mean that these two are identical? Are they the same? Well, the answer is no. They're not necessarily the same. I guess they could be the same, right, but they may not be the same. And you're thinking, well why? They have the exact same H and N. How could they be different? Well, remember that there's a lot of RNA in here. There are eight segments of RNA. Eight RNA segments. And H and N are just two proteins. There are other proteins that this virus has inside of it that might distinguish these two from each other. And so if this is the newer one, you might say, well, this is the novel H1N1 to distinguish between the two different viruses. So whenever you see words like novel H1N1, now you understand why they're coming up with that name. They're just trying to help you distinguish new from old. So actually what I did is I went ahead and made a grid. I made a grid of the H types, and the N types, all 17 H's and all 10 N's. And what I wanted to do is show you that you can actually make this really simple. You could say, well, obviously if you have this first H and this first N, then in this grid I would type in H1N1. This is where H1N1 would go. And at the other end of the grid, let's say down here, you could actually say, well, this guy down here at the H17N10. And I could do this for all 170. I could go ahead and name all 170 using this grid. So out of these 170 combinations, the one that humans care most about are the ones that most commonly affect us. And it turns out that H3N2 and H1N1 are two that are dominant in human societies, human populations. Now this wasn't always the case, but at the moment in 2013, that is the case. Actually back, let's say about four or five decades ago, a more common one was H2N2. In fact, H2N2 caused lots and lots of disease. Many people got sick from H2N2, and it was a cause of a pandemic. But since then, H2N2 has kind of been replaced by these that we see now. So I'm going to erase H2N2 entirely. Because nowadays, we see more of the H1N1 and H3N2. Now, why do you suppose that would be the case? Why would some be more dominant, or more common, in human populations over others? I mean, there are so many combinations you could think of, right? Why did these two do such a good job? Well, the answer is that from a virus's perspective, if it's trying and get to as many people as possible, it's got to do a really good job of transmitting from one sick person to another, so that it can spread really, really quickly through a whole population. And both of these viruses do that. They actually spread from person to person really effectively. So if you have the flu, and we know that it's a type A-- I should have written that earlier, because this naming system, we said, is only for type A's-- if you know you have a type A flu, the chances are pretty high that you have either one of these-- either H1N1 or H3N2. Now, with all these other possibilities, where do we see all these other H and N viruses? Well, it turns out that, unfortunately for the birds, a lot of these viruses affect them. So birds are actually where you find a lot of these viruses. Now, here's my bird. And it turns out that almost every virus that we've found can be found in a bird. And these are pigeons, ducks, you get the idea. There's only one virus, actually-- or one type of HN combination, I should say-- that you don't really see in birds. And that's this one, this H17N10. Interestingly, this one is seen in bats. So all of the other combinations are seen in birds. And in fact, not just birds. Some of these you'll see in-- let me write out here-- horses. Some of them you'll see in pigs, you'll see in dogs, and birds as well. So a lot of different animals can get these other HN type viruses. But again, just to stress the point, the ones that we see in humans are usually H1N1, or H3N2. Now, let's say that you do work with birds, that's part of your job. And one day you actually see that there's a sick bird, and unfortunately for you, you pick up the flu from it. And this could be one of the other H combinations, HN combinations. So maybe pick up H5N1 from this poor, sick, little bird. Or maybe you pick up H7N2. This has all been actually shown, that these viruses can spread from birds over to humans. And so maybe you pick up H7N3, H9N2. There are a few of them, right? If you pick up one of these viruses, then you would get sick, obviously, because you got the flu. So it does cause symptoms in humans. But the key idea is that, at the end of the day, it doesn't circulate. These viruses don't seem to circulate as well between humans. And as a result, the dominant viruses still remain these ones that actually do a better job of going from person to person-- these two human viruses, or these two dominant human viruses. I'll write "dominant." Because again, you can get sick with some of the other ones, but these are the ones that we most often see in human populations. So the last thing I want to mention is the naming structure. It gets a little bit fuzzy and confusing, because sometimes we actually name things like "avian." Sometimes you might see "avian," which is another word for "bird." Or you might see "swine," which is another word for "pig." So if you hear the terms "swine flu" or "avian flu," what do they mean? Well because RNA pieces get shuffled back and forth between birds, and humans, and pigs, from time to time, what they do is they basically try to identify, where did the genes come from for this particular virus? Did it come primarily from a bird, or from a pig? And based on what they find, they think, OK, we'll let's call it "avian flu," or let's call it "swine flu." But honestly, I think those terms are very confusing for a lot of people. And it's probably easier to just think about them in terms of H's and N's.