I am curious, is there any known disease/infection that is very severe normally (patient suffers greatly and die easily without medical treatment), but ends up having little to no effect on the lives of asymptomatic carriers? I am asking this as I am thinking whether this means certain disease symptoms are mechanisms resulted from evolution to protect the "greater good", i.e. killing the patient to avoid transmission to another member of the species.

  • $\begingroup$ I think this question is probably strongly misguided. For now I've given it a downvote; better research and focus might encourage me to change my mind on that. But as-is, it seems too contaminated with misunderstanding of both evolution and disease. $\endgroup$ – Bryan Krause Feb 19 at 5:36
  • $\begingroup$ Research needed aside, in what way it is misguided? Isnt it just asking for a possibility (a possible example and possible reason behind)? And I am not quite sure I like the way you are judging it by "probably" and "seem", without giving any actual reason. $\endgroup$ – y chung Feb 19 at 6:22
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    $\begingroup$ @ychung What I meant is that there are underlying assumptions, not contained in the question, that are wrong. One important one is how evolution works, which has now been covered a bit in the answer by user1850479. Another is the relationship between symptoms and transmission. An asymptomatic individual may carry a disease longer, but that doesn't mean they transmit it more than someone symptomatic, even if that person dies. $\endgroup$ – Bryan Krause Feb 19 at 18:01
  • $\begingroup$ You are generalizing my view, fighting a strawman. I said "certain" symptoms (in response to a particular infection/pattern of infection) that make an individual suffer and die, could have evolutionary advantage in stopping an infectious disease that could harm the greater good of the species. $\endgroup$ – y chung Feb 19 at 18:08
  • $\begingroup$ @ Bryan I really dont get why you guys are saying my understanding of evolution is wrong? So, how are you disproving that it is not possible certain mechanism could develop within the population to stop infectious disease while sacrificing individuals (as a consequence I insist)? $\endgroup$ – y chung Feb 19 at 18:09

I am curious, is there any known disease/infection that is very severe normally, but ends up having little to no effect on the lives of asymptomatic carriers?

HIV is an obvious example, where so-called "long-term nonprogressors" successfully suppress HIV replication sufficiently that they do not develop AIDS but are unable to clear the infection entirely [1].

patient suffers greatly and die easily without medical treatment

Since the comments below seem to suggest that HIV does not cause death without medical treatment, I'll point out that this is incorrect. Over 30 million people have died from HIV infection.

I am asking this as I am thinking whether this means certain disease symptoms are evolutionarily wired to protect the "greater good"

It does not. Selection happens on individuals and not whole species, so the greater good is irrelevant. Asymptomatic infection is a point on the spectrum of possible responses to infection between sterilizing immunity and death. Individuals who are chronically infected mount enough of an immune response to control the infection but not enough to completely clear it. In the case of HIV, they are typically people who have exceptionally strong immune responses to a normally lethal virus.


The comments below suggest a deep misunderstanding of evolution. Since I think these are really the core of your question, I'll address them here.

If traits really evolve only according to how much an individual reproduce sucessfully, it would be a different world today.

Which individuals reproduce is the sole determinant of which genes are passed down to the next generation. This is because the genetics of each individual are determined at conception, and cannot be changed. Hence, reproduction alone determines the traits of the next generation.

To quote wikipedia[2]:

Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation. Evolution occurs when evolutionary processes such as natural selection (including sexual selection) and genetic drift act on this variation, resulting in certain characteristics becoming more common or rare within a population

Evolution is the process by which some traits are passed down to the next generation while others are not though the mechanisms of selection (e.g. individuals with harmful traits dying) and genetic drift (e.g. random chance).

Thank about a hypothetical super fast reproducing bacteria, using up all resources, which means extinction at the end, such traits would not be passed on. In fact some sort of "moderation" mechanism is needed to take care of the "greater good".

This is exactly what happens if you put a lot of fast reproducing bacteria in a closed environment with limited resources. They will all eventually die.

You seem to be thinking of evolution as something that intelligently looks out for the wellbeing of species. This is not the case. Evolution is a random process by which different amounts of reproduction and survival between individuals alters the composition of the succeeding generations. As a random, emergent process it has no goals, no intelligence, and no sense of "good". Rather, it simply happens as a result of individuals reproducing. This frequently does lead to extinction, as evidenced by the fact that the vast majority of species no longer exist.

(1) https://academic.oup.com/cid/article/51/2/239/303856
(1) https://en.wikipedia.org/wiki/Evolution

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  • $\begingroup$ Thank you for the answer! HIV is an interesting case definitely. However, the severity of AIDS as I understood come from accumulating infections from other pathogens, not the immune response/symptoms to HIV itself which is kind of chronic (I may be wrong). I also disagree with the notion "Selection happens on individuals" makes greater good irrelevant. Even if a stressor act on individuals apparently, it could produce a trait that improves greater good. You can argue the evolution of "dying" is exactly for the greater good. In some diseases, acute symptoms can definitely make you "die faster". $\endgroup$ – y chung Feb 19 at 3:30
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    $\begingroup$ @ychung Long-term nonprogressors are so called because they do not progress to AIDS. $\endgroup$ – user1850479 Feb 19 at 4:12
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    $\begingroup$ @ychung The greater good is irrelevant to selection. Traits may or may not improve whatever you consider to be "good", but selection does not care about that. Some more reading about what selection is may help you understand that point better. $\endgroup$ – user1850479 Feb 19 at 4:14
  • $\begingroup$ My first point is trying to say that HIV/AIDS is not really what I am looking for in the question (thank you for the answer tho). Second point I disagree. Just think about it, traits can definitely be selected in the direction to preserve greater good in the face of large-scale and amplifying stressor (based on population size). Whether "selection does not care about" or not is not important, the consequence is. $\endgroup$ – y chung Feb 19 at 4:25
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    $\begingroup$ @ychung Edit your question to clarify what you're looking for. Second point: selection is the process by which individuals either reproduce or do not reproduce. A trait might act on the greater good by accident (see genetic drift), but such traits cannot be selected for. Since you have questions, what selection can and cannot do may be a good topic for a second question. $\endgroup$ – user1850479 Feb 19 at 4:58

"Yes" is the simple answer to the first part of your question. HIV, "Typhoid Mary", maybe Covid19, and other examples have been given.

The second part of your question,

I am asking this as I am thinking whether this means certain disease symptoms are mechanisms resulted from evolution to protect the "greater good", i.e. killing the patient to avoid transmission to another member of the species.

is good, but doesn't have a definite answer as far as I know. However, your idea is certainly plausible. Although displaying symptoms (e.g., a rash, cough, odd behavior, or bad odor) will not benefit the individual, it will benefit individuals who are repelled by those symptoms. In cases where individuals who have a tendency to be repelled by the symptoms also have a tendency to display those symptoms (and those tendencies are encoded genetically), close relatives of those individuals have a distinct selective advantage over other individuals in the population who do not have a tendency to be repelled.

A lot of literature relating to this idea can be found by searching "evolution of altruism" and "kin selection".

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  • $\begingroup$ Thank you for your answer! Displaying horrible symptoms to repel other individuals in the species is definitely one plausible mechanism that can reduce the transmission of pathogens. $\endgroup$ – y chung Mar 3 at 17:07
  • $\begingroup$ Take note though that the two (displaying symptoms and being repelled by the symptoms) need to become coupled through selective processes, of which kin selection is the most effective. $\endgroup$ – S. McGrew Mar 3 at 18:28
  • $\begingroup$ Here is an example of the opposite: tuberculosis apparently evolved the ability to enhance its own spread by inducing symptoms: coughing. m.medicalxpress.com/news/… $\endgroup$ – S. McGrew Mar 5 at 22:43
  • $\begingroup$ Yeah, that's why I said symptoms killing the patient (which in itself constitute a severe warning to other members of the species) is a more "direct" mechanism to stop transmission. $\endgroup$ – y chung Mar 6 at 22:45
  • $\begingroup$ That's not quite what I meant. It can be a co-evolutionary process. A microbe can evolve to cause symptoms that help spread it; a species can evolve to respond to a microbe by displaying scary symptoms; it's even possible for a species to respond to the microbe by suppressing the symptoms that help the microbe spread. $\endgroup$ – S. McGrew Mar 6 at 23:59

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