Flu shift and drift | Infectious diseases | Health & Medicine | Khan Academy

Let’s say there are
two communities, an orange community
and a purple community, and they’re separate
from each other. And your job is to go
into these communities, and find out what the
most common influenza type is that’s circulating
among the people. So you do this,
and the first thing you discover is something
that’s pretty interesting, which is that, in
the orange community, turns out that they really only
have influenza Type A. Remember that there are three
types of influenza, and, over here, the only one
that seems to be affecting people is Type A. So let me
actually write that over here, Type A. And if you go over to
the purple community, you actually find
quite the opposite. You find that over here, people
are also getting the flu, but it’s always because of Type
B. So these people over here are having influenza Type
B. And influenza Type B also has eight strands of RNA. And let me write in
purple then, Type B. So that’s what you learn in the
first kind of day on the job. Now there are many
different types of Type A that are affecting
the orange community, and what I’ve drawn for you
is just the dominant strain. So there may be a handful
of Type A’s affecting the orange people, but this
is the dominant strain. And you know, actually
the same is true over here in the purple community. They have a few different
Type B’s circulating, but the dominant strain is the
one that I’ve drawn for your. So now, let me make a
little bit of space, and let me tell you what
you’re going to have to do. Over the course
of the next year, over the course of
the next 12 months, you’re going to actually have
to follow these two communities. And what you’re going
to do is basically track out over that year
what’s happening with the dominant strain. So that’s all we care
about– not all the strains, but just the dominant strain. And you want to know how
genetically different is it compared to what it was
like on day one of your job? So when I say
genetic change, I’m really comparing
it to what it was like on the first day
of your job– comparing to initial strain. So, over the 12 months,
you’ll get a real good idea of how much change happened
while you were on the job. So let’s say you
start out, and you live close to the purple people. So, of course, initially,
there’s no change. You’re doing the Type B strain,
and you’re saying, well, yep, it hasn’t changed yet. But some time passes. Let’s say you spend some
time away, and you come back, and you visit the
purple community. And you ask them, hey,
what is the common Type B strain that you guys
are seeing nowadays? And they say, well,
it’s basically the same as it used to be, it hasn’t
really changed a lot, but there are two point
mutations that have happened. So the dominant strain now has
a couple of point mutations, so it’s a little bit
different than it used to be. And you say, a-ha, there is some
genetic change happening here. The dominant strain is
changing a little bit. And then you go, and you
visit again sometime later, and they say, yep, thanks
for visiting again. A couple more
changes have happened since you last were here. And you say, ah, interesting. Let’s plot that a
little bit higher. So now the virus,
the Type B virus, is looking slightly
different from how it was when you first
started the job. And you keep going
with this process, and you know, there’s a mutation
here, another one over here. So mutations kind of pile up. And basically what you get
is kind of a staggered line– something like this, where
it kind of goes like that, all the way to the
end of the year. So the end of the year
comes, and you look back at your virus, and you say,
ah, there are a few mutations. It’s a little
different than what it was like when I started. And those little mutations you
can see with the yellow x’s. So what would we
call this process? We call it genetic drift. This is genetic drift. This is kind of the
normal process that happens with many, many types
of viruses and bacteria. Really all viruses and
bacteria make mistakes when they replicate,
and so you’re going to see some degree
of genetic drift over time. So now here’s the cool part. You go to the orange
community, the orange county, if you want to call it that. And you say, hey, I’m here
to do the exact same thing with your Type A
influenza virus. And, in the beginning, of
course, it’s not any different. But you come back
a little bit later, and you notice that this one
has had a couple of changes, a few mutations, just
like you saw before. So you say, OK,
well, so far so good. It looks like it’s
a little changed. And then, you find
out that, you know, there’s one more mutation, when
you come back on another trip. So you say, OK, looks like
it’s a little changed further. And then, a really
interesting thing happens. What you find out
is on a third trip, that this entire
segment is gone, and it’s replaced by this. So you see a huge,
new chunk of RNA. So how do you plot that on
your genetic change axis? Well, it’s really
different, isn’t it? So you’d say, OK,
well, gosh, now that 1/8 of the entire
thing is different, that would be
something like this. That’s a huge jump. So you’d say, OK,
well now there’s been a huge genetic change. And then, you come
back on another trip, and you find out that
there’s a little mutation in this green RNA, and
maybe one over there. So, again, you’ve got
a little bit of change. And you go, and you find out
that there was another mutation here, maybe one over here. And so, you keep
plotting– you’re very loyal to your
job– you keep plotting. And then, it turns out that
there’s another big shift. Let’s say this piece gets
changed out for this one. And so, again, you
have a big, big jump. Something like that. And finally, by the
end of the year, it kind of goes up
again, because you’ve got a couple more mutations. So let’s say, there’s another
mutation there and there. So that’s what it looks like. Right? The genetic change over time
for the orange one, the Type A, is actually looking
quite different. And this one
actually has elements of what I would call
genetic drift and shift. And, more
specifically, this part would be kind of a big shift. This is where a whole chunk of
RNA got kind of incorporated into the dominant virus. These are two shifts that
might have happened that year. And these other parts– let me
circle with a different color, let’s say, over here–
this and this is actually looking more similar to
what we talked about before. These are just kind
of steady changes, steady mutations over time. And this is kind of what we have
come to know as genetic drift. So with Type A influenza,
done in orange, you can see that there is some
drift and some shift happening. And with Type B influenza,
there’s only genetic drift. Now what happens, and this
is kind of the scary part about Type A influenza,
is that whenever you have these giant
shifts, there are two here, whenever you have these
shifts, the entire community hasn’t really experienced
that new Type A influenza. They’re not used to it. Their immune systems don’t
know how to deal with it. And so, many, many
people can get sick. And what we call
it is a pandemic. So in the past, we’ve
had a few pandemics. And each time, it’s usually
because of a big genetic shift that happens, and many, many
people, as I said, get sick, go to the hospital,
and can even die.


  1. The shifts must have occurred due to another type mixing in. Perhaps someone in orange village has a sick farm animal?

  2. In addition to the comment below about shift being due to transmission from an animal and then mixing up to make a new strain, it is important to mention that when many people get sick in a short period of time in a population it is an epidemic but when this flu virus spreads world wide we can call it a pandemic

  3. Exactly right, someone in the orange village may have had a sick animal. A different Type A flu (with green RNA) was circulating among the animals or perhaps a different Type A flu (with green RNA) was circulating among some of the humans but wasn't the dominant one in the community.

  4. awesome videos …. but pls, number the videos which are at the same list because some comes before the other and i become confused
    Ex: the video "Catching and Spreading the Flu" and. "Flu Shift and Drift " was uploaded on 11 Jan but you listed the first as 2nd and the other as 10th … and i think these videos are a series and when i watched them as listed i become confused and not understand as some comes before other

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