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My understanding is that diseases tend to naturally select to be more contagious but less harmful. The more contagious more harmful mutations tend to die out

My understanding is that diseases tend to naturally select to be more contagious but less harmful. The more contagious more harmful mutations tend to die out because they incapacitate the host before maximally spreading vs. less harmful mutations. Not to say it's not possible for a more harmful mutation to spread widely, just that it's not the expected evolution.

VirusNewbie 5y ago
In a vacuum this isn't true: diseases will select to be more transmissible, that can mean less deadly, but not always, think HIV. If the incubation time is long enough before death, then it could not matter. So, you could imagine a nightmare scenario where some disease selects to have a longer incubation time while still transmitting, but then causes more severe disease down the road. However, because we have vaccines, I do think ultimately you're right. If it turns out that omicron (or another variant) is basically a cold but also evades current vaccines, I think society will 'allow' it to become endemic.
- QuarterReptile 5y ago
  On your first paragraph, I agree entirely. Regarding the second: with the vaccines only preventing or reducing severe symptoms without seeming to reduce viral load, they slow the progress of respiratory viruses toward low-impact variants. The mechanism of this is that sick people stay home and consequently, if you are a viral variant that makes people sick, you get outcompeted by variants that allow people to go about their days and keep spreading. In this sense, low vaccination rates of Africa may be just what we needed.
- murderfs 5y ago
  > In a vacuum this isn't true: diseases will select to be more transmissible, that can mean less deadly, but not always, think HIV. Is there any evidence of diseases that have selected to be less deadly, aside from by killing all of its especially vulnerable hosts? As far as I can tell, this is naive optimism: the selection pressure for not killing your host is irrelevant if they're only going to be infected for a week or two anyway, and basically every mutation that's going to increase transmissibility is likely to be negative (increased viral load in infectious agents, immune escape, etc.).
  - QuarterReptile 5y ago
    >Is there any evidence of diseases that have selected to be less deadly.. It sounds like you're asking for specific examples of pandemics that didn't happen because they weren't detrimental enough to endanger people. That category of respiratory viruses is often broadly described as "the common cold."
    - murderfs 5y ago
      No, I'm asking for specific examples of deadly viruses that have become less deadly over time. Like, take smallpox as an example. It's probably been endemic in humans for thousands of years: what's happened to its mortality rate? It's still at 30%, and the reason it's so low is due to human adaptation: the native Americans were virtually wiped out by it.
      - QuarterReptile 5y ago
        > It's probably been endemic in humans It probably was endemic, as it was declared eradicated in 1979 by an effective vaccination campaign. More importantly, though, smallpox, as a complex DNA virus, had far fewer viable mutations. That fact, along with an apparent lack of non-human reservoirs, is probably why it could be eradicated.
  - sgtnoodle 5y ago
    This article claims that was the case for H1N1, as well as the "common cold". https://www.gavi.org/vaccineswork/will-covid-19-evolve-be-more-or-less-deadly https://www.gavi.org/vaccineswork/will-covid-19-evolve-be-mo...
- m00x 5y ago
  HIV and coronaviruses are completely different and can't be compared. HIV is clinically a mild virus and doesn't cause any severe symptoms by itself. It is only when HIV becomes AIDS that it is dangerous, but then again, the symptoms are not caused by HIV/AIDS itself, but by the lack of immune cells caused by AIDS. You could have HIV forever and not even notice it if you were in a vacuum as you say :) We can't know how severe Omicron will be, but let's hope it just becomes a cold. As OP said, the most enduring parasites (viruses are categorically parasites), are the ones that don't kill their hosts. This is not only true for viruses, but all kinds of parasites.
  - Hypx_ 5y ago
    That’s not actually true. Many AIDS patients will die of cancer due to a lack of immune system suppressing those common occurring cancers.
    - m00x 5y ago
      Cancer is an additional disease, and not caused by AIDS itself. It could happen regardless of external factors, so I guess my "vacuum" analogy wasn't perfect. All I mean is that HIV/AIDS itself doesn't kill you by itself.
      - Hypx_ 5y ago
        Not quite. The body would natural develop several types of cancer without an immune system to suppress them. People with AIDS will have short lives even without exposure to any external diseases.

VirusNewbie 5y ago

In a vacuum this isn't true: diseases will select to be more transmissible, that can mean less deadly, but not always, think HIV. If the incubation time is long enough before death, then it could not matter. So, you could imagine a nightmare scenario where some disease selects to have a longer incubation time while still transmitting, but then causes more severe disease down the road. However, because we have vaccines, I do think ultimately you're right. If it turns out that omicron (or another variant) is basically a cold but also evades current vaccines, I think society will 'allow' it to become endemic.

QuarterReptile 5y ago
On your first paragraph, I agree entirely. Regarding the second: with the vaccines only preventing or reducing severe symptoms without seeming to reduce viral load, they slow the progress of respiratory viruses toward low-impact variants. The mechanism of this is that sick people stay home and consequently, if you are a viral variant that makes people sick, you get outcompeted by variants that allow people to go about their days and keep spreading. In this sense, low vaccination rates of Africa may be just what we needed.
murderfs 5y ago
> In a vacuum this isn't true: diseases will select to be more transmissible, that can mean less deadly, but not always, think HIV. Is there any evidence of diseases that have selected to be less deadly, aside from by killing all of its especially vulnerable hosts? As far as I can tell, this is naive optimism: the selection pressure for not killing your host is irrelevant if they're only going to be infected for a week or two anyway, and basically every mutation that's going to increase transmissibility is likely to be negative (increased viral load in infectious agents, immune escape, etc.).
- QuarterReptile 5y ago
  >Is there any evidence of diseases that have selected to be less deadly.. It sounds like you're asking for specific examples of pandemics that didn't happen because they weren't detrimental enough to endanger people. That category of respiratory viruses is often broadly described as "the common cold."
  - murderfs 5y ago
    No, I'm asking for specific examples of deadly viruses that have become less deadly over time. Like, take smallpox as an example. It's probably been endemic in humans for thousands of years: what's happened to its mortality rate? It's still at 30%, and the reason it's so low is due to human adaptation: the native Americans were virtually wiped out by it.
    - QuarterReptile 5y ago
      > It's probably been endemic in humans It probably was endemic, as it was declared eradicated in 1979 by an effective vaccination campaign. More importantly, though, smallpox, as a complex DNA virus, had far fewer viable mutations. That fact, along with an apparent lack of non-human reservoirs, is probably why it could be eradicated.
- sgtnoodle 5y ago
  This article claims that was the case for H1N1, as well as the "common cold". https://www.gavi.org/vaccineswork/will-covid-19-evolve-be-more-or-less-deadly https://www.gavi.org/vaccineswork/will-covid-19-evolve-be-mo...
m00x 5y ago
HIV and coronaviruses are completely different and can't be compared. HIV is clinically a mild virus and doesn't cause any severe symptoms by itself. It is only when HIV becomes AIDS that it is dangerous, but then again, the symptoms are not caused by HIV/AIDS itself, but by the lack of immune cells caused by AIDS. You could have HIV forever and not even notice it if you were in a vacuum as you say :) We can't know how severe Omicron will be, but let's hope it just becomes a cold. As OP said, the most enduring parasites (viruses are categorically parasites), are the ones that don't kill their hosts. This is not only true for viruses, but all kinds of parasites.
- Hypx_ 5y ago
  That’s not actually true. Many AIDS patients will die of cancer due to a lack of immune system suppressing those common occurring cancers.
  - m00x 5y ago
    Cancer is an additional disease, and not caused by AIDS itself. It could happen regardless of external factors, so I guess my "vacuum" analogy wasn't perfect. All I mean is that HIV/AIDS itself doesn't kill you by itself.
    - Hypx_ 5y ago
      Not quite. The body would natural develop several types of cancer without an immune system to suppress them. People with AIDS will have short lives even without exposure to any external diseases.