Naming the flu: H-something, N-something | Infectious diseases | Health & Medicine | Khan Academy


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.


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