What is the flu? | Infectious diseases | Health & Medicine | Khan Academy

– [Voiceover] So if you’re
watching this video, I’m pretty sure that at
some point in your life, or in the lives of someone you know, that you’ve had the flu. Almost everybody is pretty familiar with how awful it feels
to get sick with the flu. And when people talk about the flu, they kinda talk about it
in two different ways. And we’re gonna get into
both of those ways right now. So sometimes, they’ll
talk about the illness, or the symptoms they had
when they had the flu. And other times, they’ll
talk about the virus, you know, the actual
virus that causes the flu. So I’m gonna actually
break it down the same way, we’re gonna talk first about the illness, and then we’re gonna get into the virus. I’m gonna draw a nice
little line down the middle so that we don’t get lost
in this conversation. So I can’t help it, but any time I hear that someone has the flu, I immediately get into this mode where I have a bunch
of questions for them, and I really just wanna
kinda convince myself that when they say they have the flu, they really have the flu and
they don’t have something else. So the questions that pop up in my head, the first one is usually, is it abrupt, or did it kinda start abruptly? I’m gonna just write that. Was it abrupt? And you know, I might say, do you remember feeling well one evening, and then waking up sick. Or something like that. Do you remember exactly
kinda when it started. And most people with the flu can tell you, within a day or two, exactly when their illness started. Now another clue that something is the flu is that it usually lasts for
about three to seven days. So if someone is saying, well, I got sick with the flu, and then four months later, you know, I started getting better, that’s a very weird story. Usually it would be a
lot quicker than that. So always think around
three to seven days. Of course it could be a
few extra days than that, especially when you’re thinking about a symptom like the cough. But generally speaking,
that’s the time window. Then I really get into
the symptoms themselves. I want to find out exactly
what made them feel so sick. And there are two categories of symptoms. The first is respiratory symptoms. I want to know about respiratory symptoms. And secondly, I want to know about what I call constitutional symptoms. Constitutional symptoms. And these, this has nothing
to do with the Constitution, but it has to do with the body. Kinda thinking about symptoms
that affect the entire body. So let me make a little
bit of space on the canvas. And we’ll first start out thinking about the respiratory side. So respiratory symptoms, let me draw out air coming into the body. Remember, air has two
major paths into your body, it’s gonna come in through your nose, or it’s gonna come in
through your mouth, right? And when it comes in, it’s
going to quickly join up. Remember, air in the nose is gonna meet up with air in the mouth,
and it’s gonna go down through the windpipe, or
the trachea we call it, and it’s gonna branch off into the right and left lung. So this is my right lung over here, and on the other side
we’ve got the left lung. And remember the left
lung is also gonna be right next to the heart, so we’ve gotta keep a
little space for the heart. So these are the two lungs, and this is the air coming in. Now if someone says that
they have a stuffy nose, that’s a really common one, right? They say they’re congested, or sometimes they might say a runny nose, but any of these kinds of symptoms you can think in your mind of this picture and you can say, well yep, this is right along the path that air is gonna take on
its way into the lungs. So this is part of the respiratory track. Now, another symptom
might be a sore throat. A sore throat. And here again, you can see that the air is actually gonna be passing right through the throat on its way down into the lungs. And a really really common symptom, you hear this all the time, is cough. And whenever people talk about cough, I just usually think about the
lungs being involved there. So these symptoms, stuffy
nose, sore throat, cough, you can think of this picture and really can visualize the air going through, and somehow these parts
are being involved. Now exactly what’s happening is that the virus is of
course coming in, right? It’s being breathed in, that’s
how we take in the flu virus. And as it hits these different areas, the cells are getting damaged. And that’s what we experience as a stuffy nose or a
sore throat or a cough. It’s really damaged cells that are being affected by the virus. Now moving on to the other side, the constitutional symptoms, these are symptoms that
I want you to think of as affecting the whole body. So if this is the whole body, let’s say these are the arms and the legs. These are symptoms like fever. Now if someone says they have fever, it’s very hard to point to
one exact part of the body that’s affected, right? You might say, well, I feel feverish, really all over, and if you’re feeling feverish, this face would be very sad like that. So if you have symptoms like fevers, and with fevers oftentimes
you get chills as well. I’m gonna put that together. If you have fevers or chills, that would be a constitutional symptom. Another one would be
something like bodyaches. You know, if you are in bed because your whole body is aching, you wouldn’t point to any one place, you’d say, well it’s
just kind of all over. And so that would be another
constitutional symptom. Another one that jumps to mind is fatigue. Kinda the same idea. Where your whole body is affected here. So again, when someone tells me about flu, I’m thinking that they better have at least one of these respiratory symptoms and at least one of these
constitutional symptoms. So I think of them in
two categories, right? And I want at least
one from each category. And they must have both if they’re gonna get me convinced that they have the flu. So scrolling up just a little bit, just to make sure we don’t forget. It’s gotta be abrupt, it’s gotta happen over the course of three to seven days, and they should have at
least one respiratory symptom and at least one constitutional symptom. Now this is good if I’m
taking care of patients, or I’m thinking about a clinical setting, like the hospital or the emergency room. But what about if you’re doing research? Well, it turns out that the
Centers for Disease Control, and they do a lot of research
around influenza and flu, they have a definition that
I want you to know about. Their definition, and you’ll see this, is that the person has
to have a sore throat or a cough, one or the other, and I’ll put a big or here. And they have to have fevers. So this is very similar to the
definition I just gave you, but this is the definition they use when doing research, and when
actually presenting data. So it has to have these two things, right? Sore throat or a cough, and fevers. And so this is the definition
for influenza-like illness. Influenza-like illness. And the short way of
saying all this is ILI, influenza-like illness. So this again is the definition from the Centers for Disease Control. So if you ever hear ILI,
at least now you know what they’re considering as
being someone that has ILI. So this brings up kind of
an interesting question, and that’s why I kinda started out by splitting things up between
the illness and the virus. I wanna go over to the virus side. And just keep in the back of your mind this idea of ILI. Now on the virus side, let me just kinda sketch out very quickly in this green color, what the influenza virus looks like. I’m gonna label this influenza over here. So this is my influenza virus, and this little virus has got some RNA on the inside of it, so it’s got RNA in here. And I’m gonna draw out the RNA, just so you can see it. I’m gonna use two different colors, so let’s use like a purple color here, and one important thing about this RNA is that it’s broken up into little pieces, the way I’m drawing it here. So it’s got some purple chunks, and let’s draw some yellow
chunks of RNA as well. And this RNA of course is,
as I said, genetic material, so it’s gonna be coding for proteins. So you’ve got some proteins, and let’s say that you’ve got some proteins out here on the surface. And, you know, you
could imagine that those yellow proteins come from one
of those yellow RNA segments, and you’ve also got some
purple proteins over here, some purple proteins. I’m gonna draw out some
purple proteins here for you. So let’s say that the purple protein, we can call that H, and the yellow protein,
we can call that N. So what you’re getting
here is that you’re seeing a couple of the important
parts of influenza virus. And I just wanted you to
start getting familiar with the fact that it has, you know, RNA on the inside that
is broken up into chunks, that it’s got some surface
proteins on the outside, and a couple of those important ones, we call them H and N for short. And I’ll tell you more about
them in a future video. So this is influenza virus, and now the question
is, if you have someone, let’s say I’ve got a
friend or a family member who tells me that they
had an abrupt illness that was six days, and they
had fevers and a cough. Well, it sounds like,
based on what I said, that this meets of
course the CDC definiton for influenza-like illness. And so if I tested them, let’s say I actually, you know, checked their nose with a
little nose swab and did a test, you would expect that I would
actually find influenza there. And most of the time, I would
actually find influenza there. But not always. And this is actually an important concept. That there are actually,
believe it or not, some of these little copycat viruses. I’m gonna draw a couple
copycat viruses here for you. I’m gonna write out two of them. There are actually more than two, but we’re just gonna
talk about two of them. And I’m not gonna draw them accurately, this is just kind of a
visual representation, just to kind of show you what they are and write up their names. One is called rhinovirus. Rhinovirus. And you may be aware
that rhino means nose, and actually rhinovirus
loves to infect the nose, and that’s actually why
it’s called rhinovirus. And another copycat virus, I’m gonna draw it looking
a little different, maybe a sideways looking
thing, something like this. This guy, this is RSV. RSV. And the full name of RSV is respiratory, so you know it affects the
lungs, syncytial virus. Respiratory syncytial virus. And we’ll talk about that
another time as well. But the idea here guys, is that these copycat viruses, and this is interesting, they can actually sometimes fool us into thinking that we’re
dealing with influenza. Because some of the symptoms
you get with rhinovirus and that you get with RSV end up being pretty similiar to the symptoms you get with the flu. And so as a result, we have to have some way of telling them apart. And that’s why, actually, you may have heard of the term the cold. Let me actually bring this down and actually show you now the cold and the flu side by side, and how to kind of
distinguish between the two. So when you have, let’s say the flu, we said that usually you would have some respiratory symptoms, check, and you’d have some
constitutional symptoms, check. But if you have the cold, kinda
the common cold, we call it, then you generally only
have respiratory symptoms. You don’t typically have fevers and chills and body aches and fatigue. You don’t usually have
that stuff with the cold. So that’s kinda the easy and quick way to distinguish between
the flu and the cold. And those are the questions I always ask, you know, patients of mine. I say, well did you have
body aches or fatigue? And if the answers to all those
kinds of questions are no, I’m thinking aha, this
person has the cold. But of course, this isn’t exact, right? This isn’t perfect in
that once in a while, people will actually fool you, and they will actually have
one of these copycat viruses, the rhinovirus or RSV or
adenovirus, or many other ones. And they’ll actually have
influenza-like illness. They’ll actually have sore
throat and fevers and body aches. And so this is important to know, that every single time we
clinically think someone has, you know, flu illness, doesn’t necessarily mean that
they have the influenza virus.


  1. Yes, these are specifically subtypes of Type A influenza (there is also Type B and C, but we don't use H/N proteins to name them).

  2. Viral Pneumonia is a general description of an infection involving the alveoli in the lungs caused by some virus. It could be the flu virus or some other virus (adenovirus, RSV, etc…). Hope that helps! =)

  3. Don't like the flu. That sudden headache and chills tire me out.

    Drinking lots of water is good.

  4. People keep on trusting your Government, like they really care about your health.. I will not ever get the flu shot..

  5. Yup, it seems that I'm definitely sick with the flu.

    I woke up this morning with a phlegmmy sore throat, weak body, 101.9 degree fever, and a massive headache that lasted throughout the day. After being seen by the university health center, they gave me ibuprofen.

    That did the trick for my horrible headache, but I've gotten a bit of a cough now, which I think is my body's attempt to clear out the phlegm in my throat. My throat is feeling incredibly parched no matter how much water I drink..

  6. By nature, when some types of viruses attack… u 've got flu (headache, hot,…).
    In others way, when you 've got flu. We scan your brain by fmri method.
    We coded brain signal – and when recall that.
    U 've got the same with flu.

  7. 9 days ago a fever started abruptly, it lasted around 20 hours, around 12 hours after the fever started (the next day) I called an ambulance since I felt very bad, had the fever, chills, perspiration, I was tired all the time, I had sores, dry cough and the night when I had fever, I barely had any sleep and was a bit delirious. So the ambulance came, I explained the symptoms, the Doctor listened to my lungs and said I have a "virose" I don't know how it's translated into english, but I believe it's translated as the flu, he prescribed an antibiotic called amoxacilin, a spray for the throat called green tantum, a vitamin called vitamax, and parasinus. I asked a doctor on the internet if the doctor from the ambulance did the right thing by prescribing antibiotics, and he said no, that it was a big mistake and I should go to the hospital, it's been around 9 days since it started, I have no fever now, but it's harder to breathe sometimes, I cough out mucus and today I really felt depressed.
    By the way I have sinusitis and nasal polyps, I think that made it worse. 
    Tomorrow or the next day, but hopefully tomorrow I will go to the doctor, I hope I'll get better because this depresses me so much, today I cried for no reason, I have mood swings, it really got to me.

  8. Oh god i thought i had the flue..turns out i just have a bad cold 😛 
    How am I so sure, you ask?
    Well, in the video it says you have a sore throat OR a cough.
    – I have both
    I don't have a fever or chills
    – but I do get minor body aches
    I don't have fatigue
    – i just don't have fatigue lol

  9. every two years I get the flu, or at least I thought it was the flu. I get everything else except the fever. I think I have it right now. Sore throat, cough for nearly two weeks, fatigued, my whole head hurts when I cough 🙁 have I got the flu?

  10. To: Khan Academy

    From: Vadym Vasylenko

    Dear Sir, Madam:
    My name is Vadym Vasylenko, and thank you very much for everything you do for people. I am an English teacher and a couch in the medical interpretation field, and I used some of your clips produced on both languages (Russian and English) to train myself and others in medical terminology. For example I used these two videos: https://www.khanacademy.org/science/health-and-medicine/infectious-diseases/influenza/v/what-is-the-flu and https://ru.khanacademy.org/video?format=lite&lang=ru&v=EHXRFPKNY24

    However, I need to go deeper into psychology and psychiatry including things like depression, PTSD, ect., and I did not find a clip such as this https://www.khanacademy.org/test-prep/mcat/behavior/psychological-disorders/v/biological-basis-of-depression doubed or voice overed into Russian.
    Would you please respond to me whether you have the above stated topics in psychology and psychiatry voiced over into Russian or not, and if you don't have please recommend me some other educational entity which does.

    Thank you very much for your valuable time and efforts.
    Vadym Vasylenko, 1-646-797-5722, US, +7-981-950-0905 (Russia) Skype: praxis365

  11. Lol me from Africa, i never use orthodox medicine and i never get sick, I eat medicinal foods everyday, that's how i keep fit and healthy. Prevention is better than cure

  12. i gt flu hait 5 days so far came out know where my dughter had it day before me think i starting get better actualy out bed been in bed for 3days

  13. Hey Khan Academy,
    I really like when your videos have an outline posted in the description. Sometimes, I may not need to watch the whole video, but it is really helpful if the outline of the topics covered is posted in the description box. Thanks, -A Future Biomedical Scientist

  14. Right now I have sore throat and fatigue/malaise. I also had chills and fever and last night I was so feverish that I woke up soaked with sweat. Right now sore throats is getting better though.

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