Earlier today, a friend asked me for my take on the swine flu, so I wrote a long email, decided to research it a little more to make sure my thoughts were valid, and put up an edited version of that email here. So here are my thoughts:
[disclaimer: I am a virology/cell bio/micro grad student so I have some knowledge in this area and have cited my information below; however, I do not in any way claim to be an expert on influenza. Many parts of this are also over simplified for readability.]
Why do viruses make us sick?
Let's start with a basic question because this will help you understand more later on. At the most basic level a virus is some sort of nucleic acid (a blue print) surrounded by protein, many have other accessories as well. It's only goal is to make more of itself; that's it. It really doesn't care if its host lives or dies as long as it can reproduce itself. This presents a problem because when the virus replicates itself, it shuts off the rest of the cell's function--it can't do anything besides make more virus. If the virus infects enough cells, then you've got tissue with limited function, etc. Your body takes care of this though with your immune system. Your body recognizes that there's something foreign there and begins an generic immune response to limit the infection. It then communicates through signaling pathways to recruit defenses--white bloods cells that will get rid of the infection (this is what causes inflammation). Your body can then mount a specific response to fight the specific virus. This is relatively easy for your body to do when it has encountered the pathogen (or disease-causing agent) before or something similar to it. This is why you only get chicken pox once and why vaccines work. Your body has a memory and can fight quickly and efficiently this way. However, if it is a new pathogen, it's not as easy or efficient. Viruses also have ways to trick your immune system by hiding from it or "breaking" parts of it. Different viruses have different levels of virulence (ability to cause disease)--this means that some are better at making you sick than others. A viral infection like the flu is a localized infection. This means that it infects your respiratory system in this case and then replicates there without spreading to the rest of the body; however, if the immune system doesn't tackle it and other tissues are permissive to infection, it could spread causing an even bigger problem.
But people get the flu every year...
This is true people get the flu and many people die from human strains every year (>300,000 worldwide, 10% of these in USA). However, most of those deaths occur in people >65 years old (1) and often as a result of secondary infection (eg pneumonia on top of viral infection). So if you're middle-aged and in good health, you may feel like crap for a couple weeks but your immune system can fight it off. Human flu strains are endemic in many areas of the world--this means the virus is present at a stable rate in a population. An epidemic occurs when there is an increase in this steady state. A pandemic occurs when the virus spreads across a wide region or the whole world. The 1918 flu pandemic is estimated to have killed 20-100 million people worldwide (1). But don't freak out yet... just keep reading.
So how's swine flu different?
Viruses have host specificity--they can't infect every type of animal or plant. In flu, this is due to two proteins: hemagglutinin and neuraminadase. This is where the H and N numbers come from. These two proteins are found on the outside of the virus and they attach to receptors on the surface of the cell to allow the virus to enter. If they can't attach, it can't infect. So up until now, swine flu only attached to a pig-specific receptor and couldn't recognize the human one. The unfortunate thing is: the H and N proteins can mutate and evolve really easily. That's why you have to get a flu shot every year--because they change! In the case of the swine flu, we knew about it several years ago (2) but it couldn't infect humans at the time. Now, it's evolved to be able to. These H and N proteins are main targets recognized from host immune response, so if they change significantly, you're body won't be able to mount a specific response as quickly. Also, some strains of virus are more virulent than others--this also is dependent on how well the H and N proteins can bind to the cell, how efficient virus replication is, and the ability to block the immune response, among other things. Part of this virulence with the flu can also be the way it can mix and match. It's genome is made up of 8 segments. Swine are kind of a "mixing bowl" since they have receptors that are recognized by swine, human, and avian flu viruses so all of these viruses can mix and match in a pig, leading to a much more dangerous one (2).
Another big issue right now with the swine flu, is that it is being transmitted human to human. So it's not just going from pig to human, which would keep it more contained. It's going human to human (Flu is transmitted by aersol--saliva droplets when you cough/sneeze), and given the possibility of transcontinental and international travel that makes it a lot easier to spread.
Does it live up to the hype?
Yes and no. It is a legitimate health concern and precautions should be taken (are being taken) to limit the spread of the virus if at all possible. From what I understand, they're still not sure how easily it's being transmitted and how virulent it really is. So we shouldn't ignore it and I would encourage you to stay informed. Specifically I would suggest listening to the Center for Disease Control (cdc.gov) and the World Health Organization (who.int) before anyone else; the media is having a hay day with and exaggerating it, I think. You should also take normal flu season precautions: wash your hands after using the restroom and before eating, avoid touching your face (esp your mouth) with your hands, see a doctor and stay home from work/school if you're feeling sick.
So there's my take on it. And to paraphrase my PI: I don't know if the realistic concern is really as severe as the perceived concern [by the media].
_________________________________________________
1. Taronna R. Maines, et al. (2008). "Pathogenesis of emerging avian influenza viruses in mammals and the host innate immune response." Immunological Reviews 225(1): 68-84.
2. Wuethrich, B. (2003). "Chasing the Fickle Swine Flu." Science 299(5612): 1502.
3. All general virology info came from "Principles in Virology" by SJ Flint et al. If you want the full citation, I can give it to you, but I don't have the book at my desk right now.
[disclaimer: I am a virology/cell bio/micro grad student so I have some knowledge in this area and have cited my information below; however, I do not in any way claim to be an expert on influenza. Many parts of this are also over simplified for readability.]
Why do viruses make us sick?
Let's start with a basic question because this will help you understand more later on. At the most basic level a virus is some sort of nucleic acid (a blue print) surrounded by protein, many have other accessories as well. It's only goal is to make more of itself; that's it. It really doesn't care if its host lives or dies as long as it can reproduce itself. This presents a problem because when the virus replicates itself, it shuts off the rest of the cell's function--it can't do anything besides make more virus. If the virus infects enough cells, then you've got tissue with limited function, etc. Your body takes care of this though with your immune system. Your body recognizes that there's something foreign there and begins an generic immune response to limit the infection. It then communicates through signaling pathways to recruit defenses--white bloods cells that will get rid of the infection (this is what causes inflammation). Your body can then mount a specific response to fight the specific virus. This is relatively easy for your body to do when it has encountered the pathogen (or disease-causing agent) before or something similar to it. This is why you only get chicken pox once and why vaccines work. Your body has a memory and can fight quickly and efficiently this way. However, if it is a new pathogen, it's not as easy or efficient. Viruses also have ways to trick your immune system by hiding from it or "breaking" parts of it. Different viruses have different levels of virulence (ability to cause disease)--this means that some are better at making you sick than others. A viral infection like the flu is a localized infection. This means that it infects your respiratory system in this case and then replicates there without spreading to the rest of the body; however, if the immune system doesn't tackle it and other tissues are permissive to infection, it could spread causing an even bigger problem.
But people get the flu every year...
This is true people get the flu and many people die from human strains every year (>300,000 worldwide, 10% of these in USA). However, most of those deaths occur in people >65 years old (1) and often as a result of secondary infection (eg pneumonia on top of viral infection). So if you're middle-aged and in good health, you may feel like crap for a couple weeks but your immune system can fight it off. Human flu strains are endemic in many areas of the world--this means the virus is present at a stable rate in a population. An epidemic occurs when there is an increase in this steady state. A pandemic occurs when the virus spreads across a wide region or the whole world. The 1918 flu pandemic is estimated to have killed 20-100 million people worldwide (1). But don't freak out yet... just keep reading.
So how's swine flu different?
Viruses have host specificity--they can't infect every type of animal or plant. In flu, this is due to two proteins: hemagglutinin and neuraminadase. This is where the H and N numbers come from. These two proteins are found on the outside of the virus and they attach to receptors on the surface of the cell to allow the virus to enter. If they can't attach, it can't infect. So up until now, swine flu only attached to a pig-specific receptor and couldn't recognize the human one. The unfortunate thing is: the H and N proteins can mutate and evolve really easily. That's why you have to get a flu shot every year--because they change! In the case of the swine flu, we knew about it several years ago (2) but it couldn't infect humans at the time. Now, it's evolved to be able to. These H and N proteins are main targets recognized from host immune response, so if they change significantly, you're body won't be able to mount a specific response as quickly. Also, some strains of virus are more virulent than others--this also is dependent on how well the H and N proteins can bind to the cell, how efficient virus replication is, and the ability to block the immune response, among other things. Part of this virulence with the flu can also be the way it can mix and match. It's genome is made up of 8 segments. Swine are kind of a "mixing bowl" since they have receptors that are recognized by swine, human, and avian flu viruses so all of these viruses can mix and match in a pig, leading to a much more dangerous one (2).
Another big issue right now with the swine flu, is that it is being transmitted human to human. So it's not just going from pig to human, which would keep it more contained. It's going human to human (Flu is transmitted by aersol--saliva droplets when you cough/sneeze), and given the possibility of transcontinental and international travel that makes it a lot easier to spread.
Does it live up to the hype?
Yes and no. It is a legitimate health concern and precautions should be taken (are being taken) to limit the spread of the virus if at all possible. From what I understand, they're still not sure how easily it's being transmitted and how virulent it really is. So we shouldn't ignore it and I would encourage you to stay informed. Specifically I would suggest listening to the Center for Disease Control (cdc.gov) and the World Health Organization (who.int) before anyone else; the media is having a hay day with and exaggerating it, I think. You should also take normal flu season precautions: wash your hands after using the restroom and before eating, avoid touching your face (esp your mouth) with your hands, see a doctor and stay home from work/school if you're feeling sick.
So there's my take on it. And to paraphrase my PI: I don't know if the realistic concern is really as severe as the perceived concern [by the media].
_________________________________________________
1. Taronna R. Maines, et al. (2008). "Pathogenesis of emerging avian influenza viruses in mammals and the host innate immune response." Immunological Reviews 225(1): 68-84.
2. Wuethrich, B. (2003). "Chasing the Fickle Swine Flu." Science 299(5612): 1502.
3. All general virology info came from "Principles in Virology" by SJ Flint et al. If you want the full citation, I can give it to you, but I don't have the book at my desk right now.
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