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What is the flu?

Learn the difference between the cold and the flu, and how we sometimes can get fooled by viruses! Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Rishi Desai.

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  • leaf blue style avatar for user David Elijah de Siqueira Campos McLaughlin
    What is the RNA?
    (31 votes)
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  • piceratops ultimate style avatar for user NotMyRealUsername
    My flus or sicknesses always, and I mean ALWAYS, last one day. I am so abnormal. I just learned that if you ever get the same flu, your memory cells memorize it and the b and t cells attack immediately. but before it does that, the flu mutates so memory cells can't remember it. does that just mean I pick up the flu too fast and it doesn't have time to mutate? or the virus doesn't mutate and I pick it up? or does it mean my cells are extremely good at extinguishing the virus from foreign material?
    (9 votes)
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    • female robot grace style avatar for user Hypernova
      It probably means that you have a very strong immune system. This means that the parts of your body which are devoted to fighting off sickness are very strong. You also wouldn't get many sicknesses.

      Another cause of this may be that you are not getting colds. The average cold lasts 5-7 days, and a 1-day-cold is highly irregular. You may be having allergies. Many people are allergic to pollen, dust, hay, and even decomposing leaves.
      (21 votes)
  • hopper jumping style avatar for user Zopple2010
    When I am sick and vomiting (sometimes with diarrhea, sometimes not), my mother will say I have the "24-hour stomach flu". Vomiting was not listed in the video as a symptom of the flu or the cold. What illness do I have?
    (12 votes)
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  • male robot hal style avatar for user Filipe Ronzani
    Why we have fever when we get sick?
    (14 votes)
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  • piceratops ultimate style avatar for user einstein123
    What is fatigue?
    (5 votes)
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  • piceratops ultimate style avatar for user Charles Xu
    Is the common cold a virus? What is It caused by? Is it caused by being cold? And why does being cold make you get a fever or get sick? Sorry its a lot of questions but thanks. (:-)-|<
    (7 votes)
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    • duskpin ultimate style avatar for user Aysis
      common cold is a viral infectious disease. its caused by:
      - acquiring it from another ill person
      - living in unhygienic conditions
      - sudden changes in the temperatures of ur surroundings
      - germs present on food
      and so many more ways...
      it isnt caused by being cold but feeling cold is a symptom of a common cold. when you catch a cold ur temperature rises and u get a fever because thats the natural immune response of the body. the high temperature prevents the virus from being able to survive in ur body and so kills them. normal temperature of a human is 37*C. the temperature should never exceed beyond 39*C or at the most 40*. or else u may die of overheating. this is why sweating is induced to keep the temp in check and lots of fluids are supplied to maintain water levels of the body
      (3 votes)
  • leafers ultimate style avatar for user eightsquare
    What exactly is the mechanism behind fever? Why does our body get hot?
    (5 votes)
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  • duskpin tree style avatar for user TheFlyingScotsman
    What causes the body ache ?
    (5 votes)
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  • primosaur ultimate style avatar for user AriRCAllen
    i got sick a couple days ago and my grandma told me to gargle some salt water before bed so i would'nt get a sore throat the next day and it worked, how does it work?
    (5 votes)
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    • piceratops ultimate style avatar for user Ben Burch
      Cells (bacteria included) have permeable (stuff can go through it) membranes surrounding them. When you gargle salt water, the water rushes out of the bacteria to balance out the saltiness of the water out there and the water in the cell by lowering the salt concentration. That causes them to shrivel up and die. Our cells are bigger than the bacteria's, so it takes longer for them to die.
      (3 votes)
  • female robot grace style avatar for user Chika Chukwu
    How can you get a stronger immune system?
    (2 votes)
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    • female robot grace style avatar for user Makena
      To build up your immune system's strength, you need to do a lot of exercise and eat right. (You might want to look up good eating habits for some tips.) One thing that really helps is drinking enough water every day.
      You can figure out how much water you are supposed to drink by multiplying your body weight by .70 ( .75 if you're really active or .50 if you are not.) and then dividing the result by 12. That will be the amount of glasses of water you should drink every day.
      (1 vote)

Video transcript

- [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.