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Thread: What makes some viruses more contagious than others?

  1. #1 What makes some viruses more contagious than others? 
    Forum Junior Zitterbewegung's Avatar
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    I was pondering the question: What exactly determines how contagious a virus is?
    I mean, there are viruses like measle, small pocks or rubella that are extremely contagious and would spread like wildfire without vaccination. But others like e.g. rabies or HIV require real infiltration. What's the reason behind this?


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    Google 'virulence factors.' One example is how long the virus can survive outside a host.


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    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    It is also highly dependent on the host receptor recognized by the virus. For example HIV recognizes the CD4 receptor on T cells, so it would have to come into contact with T cells to infect a person.
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    And also it would depend how much the virus has evolved/adapted. If the virus has evolved to the point where it can survive a long period of time outside the host and can be transmitted through the air then it would be allot more contagious than a virus that can only be transmitted through some bodily fluids (such as HIV)
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    In viruses, as well as with most other pathogenic organisms that live in and cause damage to their host, there is a trade-off between the ease with which the virus spreads, and how much damage they inflict on their host.

    A virus that kills its host in a few days has to be able to spread to a new host before its original host's death - otherwise, the virus will die with their host. However, a virus that allows its host to live for a very long time does not require a fast method of transmission. These mechanics of natural selection result in viruses with varying adaptations for transmission, including such things as how long they live outside of their host.

    Just look at smallpox vs. HIV. A person with smallpox that receives no treatment dies very quickly, and also spreads more quickly. A person with HIV can live for quite some time, and usually doesn't die from the virus itself at all but from their resultingly weak immune system.
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    Things start to clear up. Thanks to everyone for pointing me in the right direction.
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    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    Quote Originally Posted by paralith
    In viruses, as well as with most other pathogenic organisms that live in and cause damage to their host, there is a trade-off between the ease with which the virus spreads, and how much damage they inflict on their host.

    A virus that kills its host in a few days has to be able to spread to a new host before its original host's death - otherwise, the virus will die with their host. However, a virus that allows its host to live for a very long time does not require a fast method of transmission. These mechanics of natural selection result in viruses with varying adaptations for transmission, including such things as how long they live outside of their host.

    Just look at smallpox vs. HIV. A person with smallpox that receives no treatment dies very quickly, and also spreads more quickly. A person with HIV can live for quite some time, and usually doesn't die from the virus itself at all but from their resultingly weak immune system.
    Also, selective pressure can be applied through the immune system, if a person can fight off the virus in 1-2 weeks then the virus also needs to be able to spread quickly. This is the case with the Rhinoviridae and Adenoviridae that cause the "common cold". Or viruses like Influenza viridae undergo a lot of recombination and mutation to continuously keep ahead of the immunity of the host.

    Although, that is really addressing the reason why viruses have different infectivities, and not how.
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    Forum Cosmic Wizard paralith's Avatar
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    Quote Originally Posted by i_feel_tiredsleepy
    Although, that is really addressing the reason why viruses have different infectivities, and not how.
    I know. But the OP asked, what determines how contagious a virus is, and I think answers at various levels of biology are all appropriate. You guys covered the proximate causes well enough, so I went for the ultimate causes.
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  10. #9  
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    Quote Originally Posted by paralith
    Quote Originally Posted by i_feel_tiredsleepy
    Although, that is really addressing the reason why viruses have different infectivities, and not how.
    I know. But the OP asked, what determines how contagious a virus is, and I think answers at various levels of biology are all appropriate. You guys covered the proximate causes well enough, so I went for the ultimate causes.
    I suppose I as a student of microbiology have been brainwashed to always look at things at the biochemical level and leave the questions of evolution up to the regular biologist .
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  11. #10  
    Forum Cosmic Wizard paralith's Avatar
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    Quote Originally Posted by i_feel_tiredsleepy
    Quote Originally Posted by paralith
    Quote Originally Posted by i_feel_tiredsleepy
    Although, that is really addressing the reason why viruses have different infectivities, and not how.
    I know. But the OP asked, what determines how contagious a virus is, and I think answers at various levels of biology are all appropriate. You guys covered the proximate causes well enough, so I went for the ultimate causes.
    I suppose I as a student of microbiology have been brainwashed to always look at things at the biochemical level and leave the questions of evolution up to the regular biologist .
    Then un-brainwash yourself, my dear! I am no doubt biased since evolution is my favorite subject, but it is intrinsic to all fields of biological study. It's always good to keep in mind all levels of analysis, even if you mostly focus on one or two of them.
    Man can will nothing unless he has first understood that he must count on no one but himself; that he is alone, abandoned on earth in the midst of his infinite responsibilities, without help, with no other aim than the one he sets himself, with no other destiny than the one he forges for himself on this earth.
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  12. #11 why are some viruses more contagious than others? 
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    As far as my knowledge extent, how contagious a virus is depends on its type, and, therefore, its structure and components. The viral envelope,for example, affects greatly the virus (contagiousness). This envelope helps the virus enter the host cell by fusion of the envelope with the host cellular membrane.
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  13. #12  
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    Proximate answers actual require some knowledge, ultimate answers you can just bullshit :P
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  14. #13  
    Forum Cosmic Wizard paralith's Avatar
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    Quote Originally Posted by NeptuneCircle
    Proximate answers actual require some knowledge, ultimate answers you can just bullshit :P
    You can bullshit proximate answers too. They just won't be correct.
    Man can will nothing unless he has first understood that he must count on no one but himself; that he is alone, abandoned on earth in the midst of his infinite responsibilities, without help, with no other aim than the one he sets himself, with no other destiny than the one he forges for himself on this earth.
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  15. #14  
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    Viruses are obligate parasites - they can't reproduce outside the host. They are all contagious enough to sustain.
    I suppose your question is more limited to those that infect human and speaks to frequency of infection. There are many reasons, including infective dose, exposure, specific and nonspecific immunity of host and any viable vector, etc.
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  16. #15  
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    Looking at this from an alternative angle, i would also suggest that evolvability may have a role here, anyone else agree with me
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  17. #16 viruses 
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    well,this is an interesting question. there are some viruses that can be considered contagious.this question reminds me of one more intresting fact. which is->why have only homosapiens(humans) been able to survive evolution. i think there is some similarity between answers to these questions. some viruses infect really fast. but we cannot build a mathematical model to tell how many people are likely to get affected by any virus.if that would be possible then it would be great.wont it be?
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  18. #17  
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    I think all species currently alive have been able to survive evolution.
    "Kill them all and let God sort them out."

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  19. #18  
    Forum Junior Zitterbewegung's Avatar
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    Quote Originally Posted by spuriousmonkey
    I think all species currently alive have been able to survive evolution.
    ow do you "survive" evolution? You are subject to it, nay?
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  20. #19  
    Forum Cosmic Wizard spuriousmonkey's Avatar
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    I think it is a retarded statement to start with.
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  21. #20 viruses' evolution 
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    Does anyone know the origins of viruses. I read in wikipedia that there have not been found yet fossils with viruses. How they originated then and when? Definately after the arrival of animals or bacteria but are there theories on that?
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  22. #21 Re: viruses' evolution 
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    Quote Originally Posted by wienertakesall
    Does anyone know the origins of viruses. I read in wikipedia that there have not been found yet fossils with viruses. How they originated then and when? Definately after the arrival of animals or bacteria but are there theories on that?
    It would be impossible to ever find a viral fossil. Origins of viruses are best investigated through genetic analysis, but all that has really gotten us is families of viruses, and they seem to pick up genetic traits from their host.

    There are two possibilities, viruses are a residue of an intermediate life form, between bacteria and the origin of life. Or, they are bacteria/archaea that have degenerated as a result of a parasitic lifestyle.

    What we can say with confidence is that viruses have been around practically since the origin of life, since there isn't a single organism that does not have viruses, this suggests that viruses have been evolving alongside the traditional lifeforms for quite a while.
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  23. #22  
    Forum Cosmic Wizard spuriousmonkey's Avatar
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    Or they are descendants of the original life form.
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  24. #23  
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    Is rogue organelle possible?
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  25. #24  
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    Quote Originally Posted by Pong
    Is rogue organelle possible?
    Viruses probably predate the advent of organelles, but we don't really completely understand where organelles come from. (apart from mitochondria and chloroplast which are strongly supported by the endosymbiotic theory). Anything is possible I suppose.
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  26. #25 viruses 
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    if i am not mistaken some viruses convert carbon dioxide to oxygen. is it not so?
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  27. #26 Re: viruses 
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    Quote Originally Posted by parag1973
    if i am not mistaken some viruses convert carbon dioxide to oxygen. is it not so?
    None that I can think of... I know of no viruses which perform any metabolic activity, usually all their genes encode for capsule proteins, transcription factors, and envelope proteins with a few special exceptions with some viruses that have mhc analogues or other immune resisting proteins like apoptosis supression.
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