When flu viruses attack! | Infectious diseases | Health & Medicine | Khan Academy

So let’s talk about exactly
how flu causes so much damage to ourselves and why it
makes us feel so lousy whenever we get the flu. I’m going to start out by
drawing the flu virus here. This is our influenza virus. And we have on
influenza a couple of features we have to remember. So on the outside there’s
this little envelope, and what’s on the inside of this
envelope are eight bits of RNA. Eight pieces of RNA. And so this RNA is
important to remember, because in the human
cell, in our cells– I’m going to draw
one of our cells right here– we have,
instead of RNA, we have DNA. Remember. And so this is our nucleus, and
on the inside of our nucleus is our DNA. So this is our DNA over here. So the virus has
RNA, and we have DNA. And the outside of
the human cell– actually let me
label this over here. This is human cell. The outside of
the human cell has something called sialic acid. They’re these little strands
over here that are coming off. I’m drawing them far larger
than they are in real life. They’re not nearly
this big, but they’re these little tiny little
things called sialic acid. And this sialic acid becomes
very important in understanding how the influenza virus gets
into and out of our cells. So on the outside, remember,
of the influenza virus, there were a couple of proteins. And I’m going to draw one
of these proteins here, and I’m going to make
it look like a hand. So this is a little
hand, and this protein is called hemagglutinin. In fact, previously I had
called it the H protein, and you can call it
that if you want. But the full name
is hemagglutinin. And what hemagglutinin
does is that it actually holds onto sialic acid. In fact, that’s an easy
way to remember it, right? Because H and H go together. It holds sialic acid. And that becomes very
important, because that allows it the first step
towards getting into the cell. Now there’s another
protein on the outside here– I’m going to make it
look like a pair of scissors, because that will kind of
remind us what this one does. And this is called
neuraminidase. And I’m going to–
neuraminidase. And I’m going to
pass on explaining what it does, just
for the moment. I’ll tell you in a
little bit what it does. So then the first step
to get into the cell is for hemagglutinin to
hold on to sialic acid. And then there are a few
other small molecular steps that happen, important ones. But I’m going to suffice
to say it gets inside. And once the influenza
virus gets inside, these RNA segments,
they are let loose. So these segments
are going to start making their way
towards the nucleus. And so once they get
into the nucleus, they’re in that same kind
of area that the DNA is, and what they do is remarkable. They basically, they take over. These little RNA start making
many copies of themselves. And what they want to do is make
our human cell into a factory. They want to make a factory. And this factory is going
to make little proteins, viral proteins, and it’s
going to make viral RNA. And what it’s not going to do,
the one thing that the cell is no longer going to be very good
at doing, is its normal job. So the human cell, of
course, had some job to do. And it’s not going to have the
resources or the time to do it, because it’s basically being
taken over by this viral RNA. So what happens then is that
the viral RNA is basically turned it into a factory. And what it wants to do is make
more and more copies of itself. So let me actually just show
you what that would look like. Here’s a daughter cell. Let’s say it goes over here. And I’m going to clean
this up a little bit, just to make sure
that we’re looking at a nice, neat picture. Let’s say something like this. So this is our daughter
cell in the other side, and these cells are going
to try to make their way out of the human cell, right? Because now they’re packaged,
they’re ready to go, and where do you suppose
they want to go next? Where they’re going to want
to find their own human cell to invade, because they want
to continue this process. So we’ve got more human
cells over here down below. And we’ve got maybe, let’s
say, one human cell up here. So we’ve got new
targets for this virus, and this virus is going
to seek out these targets and try to make its way inside
again using its hemagglutinin. But before it can do that,
it’s got to break loose, right? Because it’s still attached
to that sialic acid. And so here’s where
neuraminidase comes in. The neuraminidase, it
basically, it nicks, and there’s where the N is
helpful for remembering it. It nicks, or cuts, sialic acid. And so if it can nick
or cut that sialic acid, it can break free. And so I remember the two
proteins as hemagglutinin hold sialic acid
to enter the cell. That’s on entry. And the neuraminidase is
going to nick the sialic acid, and that’s important
for exiting the cell. But we still haven’t
answered the question, how does all this
cause our symptoms? Well, what happens is that
as the cells get turned into factories, they start
dying, or getting damaged. And all their contents
start leaking out. So all these contents from
the cell start leaking out. And as they do, they
create inflammation. Let me bring up a
little bit of canvas. If you have
inflammation, let’s say that inflammation is
happening in your nose. Well, you might
say, well, I have a runny nose or a stuffy nose. Or if that inflammation is
happening in your throat, you might say, well,
I have a sore throat. It might hurt. And if it’s happening in your
lungs, you might have a cough. So a lot of those
respiratory symptoms– remember we had two categories–
a lot of those respiratory symptoms– I’m going
to shorten as “resp.”– those are going to be
explained by inflammation, or at least in part,
by inflammation. And remember there are also
constitutional symptoms, right? With constitutional
symptoms, those are things like having a
fever, or having fatigue. And the reason for that
is that your immune system is going wild and crazy. When you have
influenza, it’s going to be attracted to all
of those chemicals. We call these cytokines
that are being released. And it’s going to be
attracted to the fact that you’ve got actual virus
particles in the area that’s being infected. So that strong immune
system is going to create some of your symptoms. It’s actually going to
rev up your temperature, and you’ll start having
a fever or chills. And because all your energy
is being spent on this attack, you’re going to be fatigued. You’re going to be
fighting off the virus. Since you’re going
to be fatigued you might have body aches, so a
lot of these kinds of symptoms, you get as a result of a
strong immune response.


  1. why does one's sinus mucus go from thin and clear at the start of a virus to thick and green during the end of a cold or flu? Is that just dead cell continence?

  2. Makes perfect sense. Thanks. I am experiencing all constitutional symptoms at present and I have a sore throat. Now I know once again how my patient's feel. Being healthy is a blessing from God.

  3. How does the virus know which sialic acid to cut? What allows it to not cut the initial attachment, but cut the exiting attachment to sialic acid?

  4. I'm watching this coz I have so bad flu and wanna know what the hell the virus is which made me feel these constitutional and respiratory symptoms and really grateful for ur healthy explanation :)).. By the way, I'm getting better now :))

  5. 2:34 It cleaves sialic acid when the virus leaves the cell, allowing clean exit and more efficient viral spread within the body, because otherwise Influenza runs the risk of re-infecting the same cell and going nowhere

    oh, and spoilers

  6. I thought the Neuraminidase also destroyed the Sialic Acid receptors on the dying cell so as to avoid the new emerging Viruses attaching to it and not be able to drift off and infect fresh cells?

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