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Video transcript
There are three groups of influenza viruses, which we call virus types. And these three types are influenza A, B, and C. So we group influenza viruses like this because of the differences that they have in their genetic material or their genome. So the genome in an influenza A virus is very different than the genome in the influenza B virus, and there are many more differences between A and B than there are between two influenza A viruses. So I'm focusing right now on influenza A and B viruses, because these are the types that cause sickness and epidemics every winter in the United States. I'm not focusing on influenza type C, because this is much less common in humans and actually isn't even included in the annual vaccine. So now let's get back to the differences between individual virus types. So the influenza type A is a very large group of viruses, much larger than the type B. And so we need to further divide this group according to subtype, and the subtype is named according to surface proteins that are on the outside of every virus. So there are two kinds of surface proteins on every influenza A virus, and we call these H for hemagglutinin and N for neuroaminidase. Now, these surface proteins come in many different flavors. There are actually 17 different kinds of hemagglutinin proteins and 10 different kinds of neuroaminidase proteins. So when a virus replicates, its genome is going to determine what kinds of H and N proteins will show up on the surface of the virus. So you can imagine that there are a lot of different combinations of these viruses. So H1N1 could be a potential combination, maybe H3N2, and so on. And actually, H1N1 and H3N2 are the subtypes that we see in humans today. So the combination of proteins that we see in these viruses is very important, because this is what the immune system sees when a virus enters the body. It sees what's on the outside of the virus. So when you have two viruses that are the same or that look the same on the outside and if the immune system knows what to do with one of them, it'll know what to do with the other one. But if you have a virus come along that looks very different from a virus that the immune system has ever seen before, it's going to be very confused and not know what to do with this new virus. And when this happens, this is what causes major illness across an entire population. And we'll learn more about that in a future module.