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Let's talk about influenza viruses and the three different types of influenza that there are. There are actually three types, or three families, I think of them as. And we're going to go through each family. And we're going to talk about the differences between them. And so let's actually just label the type over here. There's type A. And this is the most common type you usually hear about. And type B and C I think are less often talked about. But we're going to go through them systematically. So the first thing I want to talk about are the symptoms. If you actually get these types of influenza, what are the symptoms you would expect? Well, for type A you expect the classic things that we talk about with flu. Some respiratory symptom, like maybe a cough. And also some constitutional symptom. I'm just going to write constitutional. Kind of a short form. Constit. And that would be something like a fever or a malaise or body ache. Something like that. And for type B, it's actually pretty much the same. Sometimes people say type B is a little bit more mild than type A. But generally speaking, it's very hard to know whether you have type A or type B flu. So these are the first two, right? These are the classic way we think about flu. Respiratory symptoms and constitutional symptoms. Now, type C is actually kind of different. It's actually usually only respiratory symptoms. Now, of course I'm not going to use the word always whenever I do this kind of thing because everyone can find an example of an exception. But type C is usually going to be just a respiratory symptom. It's usually more mild. So you might have a stuffy nose and a sore throat, but you wouldn't have the other things. You wouldn't have any of the fever, the malaise, body aches, none of that kind of stuff. Usually. So that's one key difference. As I go through I'm going to kind of circle some things I think are kind of interesting. And this is to me very interesting because here we have an example of influenza type C that's actually causing symptoms that we don't classically think of as flu. And we would actually-- if someone had a runny nose and a sore throat-- I would think that they had the common cold. So here I'm getting tricked again. So initially we talked about how you have copycat viruses. But here's an example of the opposite. Where the influenza virus actually isn't even causing what I would clinically call the flu. So what's another difference between these three types that we have listed here? Let me actually write out the term epidemic. And you may not be totally comfortable with what this word means. And sometimes people use the word differently. So I'm going to mention what I mean. But type A and type B can both cause epidemics. And type C really doesn't. And what I mean when I say epidemic. Let me actually just draw out quickly the idea of an epidemic in my mind. Let's say you have one year at the bottom. January, February, March, April, May. I'm going to go through the whole calendar year. This will be June and July. And then August, September, October, November, December. This is the calendar year. Now, if I'm thinking of type A or B, I would actually probably expect something like this. Where you have a high level of activity in the winter. And then in the summer it goes down. And then as the winter months approach again you see the activity go up. And so this would be type A and type B. Now, with type C it's actually really different. So type C I'm going to do in a red color. Usually you have a low level of activity all throughout the year. So it doesn't really change a lot. So when I say epidemic, what I'm really referring to, is the fact that you can see that there's an elevation in terms of the number of cases-- this is number of cases. You're seeing more cases during some months of the year than the baseline. It rises away from the baseline and then it dips down in the summer months. So whenever you see more cases than you would expect, we sometimes think of that as an epidemic. And in this case you might even call it a winter epidemic. So type A and B cause these winter epidemics, where more people get sick. And type C doesn't usually do that. Now we haven't talked a whole lot about it, but what about vaccine? Which of these influenza viruses can you find in the vaccine? Well, type A is in the vaccine. And so is type B. But type C is not. And this actually makes sense because with the vaccine you're really worried about people who are going to get very sick. So these are people that have fevers and malaise. And these are the people that are probably going to go on to get more sick because it's a more severe disease. And again you want to prevent as much disease as possible. So this is the epidemics that you're trying to prevent. So it makes sense that type A and type B are in the vaccine, whereas type C is not. So let me circle that. Because I think that's also an interesting and important fact about influenza. And what about the idea of genetic drift? And this alludes to the idea of mutations building up. And all three types, A, B, and C, all have mutations, from time to time that causes changes in the way the virus actually looks to your immune system. So the proteins might change a little bit. And all three of them actually-- they mutate at a different rate. So interestingly, the mutation rate is lowest for type C and highest for type A. So type A has the highest mutation rate. So this is the mutation rate creeping up. And it's interesting that it's actually quite high for type A, which is again-- that's one of the ones that's in the vaccine. Remember? And alongside genetic drift I want to mention the other one. Remember we talked about genetic drift and genetic shift. And this actually is more about shuffling bits of RNA, or pieces of that genetic material around when two viruses infect the same cell. And we know that this is a major issue when we think of type A. But this doesn't really happen in any clinically significant way for type B or type C. It's not a major issue for those types. But it is a major issue for type A. And next to genetic shift, let me write pandemic. Because this is what we always worry about, right? We don't want a virus or an influenza virus to just rip through a population and cause massive, massive numbers of deaths and hospitalizations. And again that is a concern with type A. We've seen it many times in the past 100 years or so with type A. And that's not a concern with type B or C. And it's related of course directly to genetic shift. And finally, the last category I want to write up is animals. We know all three types of influenza are going to affect humans. But which ones actually affect animals as well? Now type A, this is the one that affects tons and tons of animals. In fact, birds are probably the one that jump to mind. You always hear these words like avian flu, swine flu, right? That has to do with pigs. And there's actually also-- horses can get some of these type A's. Dogs can get them. So lots and lots of animals are affected. And I want to point out that all these animals I'm writing up here are animals that humans regularly deal with or are around. So farmers might be around pigs. And if you're into horse riding you might be around horses. Many of us have dogs. And birds are flying above us all the time. And by comparison, there really aren't any animals that humans are regularly in contact with that get type B. So that's not really an issue. Animals are not an issue for type B. And for type C, there are a couple of animals here I can mention. Pigs. Dogs. It's not as big a deal as it is for type A. So really type A-- this is an important point and it really goes together with this idea of genetic shift and pandemics. Because you remember you can get all this shuffling of genes that happened between birds and pigs and humans. And that's sometimes what sets up genetic shift. And of course if that happens you might have a pandemic. So it all goes together.