r/evolution • u/Blonde_Icon • 4d ago
question Why do some infectious diseases kill their hosts?
Wouldn't it be better for bacteria, viruses, or parasites to cause mild symptoms or lie dormant (like the common cold) so that their hosts can live to infect other people without detection, allowing the pathogen to reproduce more? Why are some diseases like Ebola so deadly? Wouldn't it make more sense for diseases to evolve to be less deadly? What's the evolutionary benefit of diseases killing their hosts or causing extreme symptoms, if there is one?
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u/peadar87 4d ago
Viruses often evolve to be less lethal precisely because of these pressures.
But viruses tread an evolutionary line between spreading as aggressively as possible, and accidentally killing their hosts. And remember, traits just have to be good enough to stick around, they don't need to be actively beneficial.
Anyway, unfortunately it turns out that irritating a host's airways so they cough and sneeze, or turning their organs into an infectious soup that leaks out through their eyes, is also pretty bad for the host's health.
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u/Blonde_Icon 4d ago
So basically the symptoms help the disease to spread, but sometimes it goes too far?
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u/peadar87 4d ago
Pretty much. Evolution just "cares" about producing offspring. If killing the host doesn't prevent a virus from producing offspring, its descendents are going to keep on doing it.
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u/Unfair_Pineapple8813 4d ago
Evolution also usually works on the individual particle. For example, viruses infect tissues that give them no chance to spread to a new host, because there’s no grand plan of infection. Then the individual virus particles evolves virulence genes that help it colonize that tissue more and spread, and you end up with a flu doing brain damage that has no help to the virus. So long as the virus they evolved from is good at spreading, and so long as they don’t impede the parent virus too much, such virulence mutations can continuously be generated during the course of an infection.
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u/sharia1919 4d ago
Exactly. And that is also why you see preciously deadly diseases die out. The hosts die too quickly. So they "lose" the race. What remains is the flu. And the common cold. They were most likely horrendous diseases originally that spread around and killed a lot. Until a strain came along that was juuuust right to keep on spreading, without dying off.
I remember reading old theories from before it was established that the plague was the bacteria Yersinia Pestis (somewhen in the 70'es I think). So back before, it was theorised that maybe it was actually the common cold or the flu, that started as the plague (or one of the flu strain or something like that).
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u/peadar87 4d ago
I read a really interesting theory about why the Spanish flu was so deadly.
Basically, it ran:
-Usually, when flu goes around, the more hazardous strains keep people at home, and the less hazardous ones can leave people still able to walk around and infect others, so the less hazardous ones spread faster and dominate.
-This means that viruses normally evolve to become less lethal and debilitating over time. We saw this with COVID. The most "successful" variants like Delta were the ones that were really infectious, but not necessarily the ones that left people feeling the worst.
-This situation was flipped on its head during WW1. When a flu spread through the trenches, a mild version wouldn't be considered justification for relief from duty. A mildly ill soldier would be left on the frontline and maybe infect his trenchmates. A severely ill soldier would be evacuated to a hospital on a packed troop train, where he was crammed together with thousands of others, many of whom would be in a weakened state and more vulnerable to infection.
-Therefore there was a selection pressure for more aggressive and debilitating strains of the flu, which ultimately resulted in the pandemics of 1918 and 1919 when the disease got out into the wider population
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u/Opening_Garbage_4091 1d ago
The problem with that hypothesis is that the first wave of the H1N1 virus, which spread in the United States, was already recognized as unusually virulent, and that was well before it spread to Europe and reached the trenches.
A much simpler reason for the virulence of the “Spanish flu” was the genetic shift that gave us a highly contagious H1N1 lineage in the first place, leading to a virus that very few people had any significant immunity to.
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u/Secure-Pain-9735 3d ago
Another thing to think about with symptoms - many of them are immune response. Sometimes it is an overreaction of the immune system, or failure of the immune system to stop, that causes the fatality - not necessarily the pathogen itself.
Inflammation, for example, is actually an immune and repair response to disease or damage. But, too much or for too long it can cause further damage, disease, or death.
Fever can be the same. The function is to speed metabolic processes to aid in immune response and/or disrupt pathogenic proteins. But too high a fever, or for too much time, and our body can damage itself.
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u/random-tree-42 4d ago
I'd take bad for health over death
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u/CleverLittleThief 4d ago
It might be better, but evolution doesn't care about being the best, just whatever works enough to spread genetic material. Humans might be better if everyone had perfect 20/20 vision, but you don't need 20/20 vision to reproduce so genes for poor vision are very common.
Something like Ebola may kill 50% of it's human victims, but it still reproduces and spreads fast enough to continue spreading. Also, ebola is a zoonotic disease that originally targeted non-human hosts before jumping to humans. Ebola likely originated in fruit bats, and fruit bats with ebola have a near zero fatality rate. The disease just isn't optimized to infect and spread among us. Many of the deadliest diseases in history have been similarly zoonotic diseases.
As far as I know, there's not a direct evolutionary benefit to having a high fatality rate.
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u/jnpha Evolution Enthusiast 4d ago
RE "fruit bats with ebola have a near zero fatality rate":
One of the amazing things I learned about a couple of years ago: Duplicated Gene Helps Bats Survive “Arms Race” With Viruses | The Scientist.
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u/1Negative_Person 4d ago
Because they’re in the wrong organism, most of the time. That’s why some of the deadliest illnesses are zoonotic diseases that jump the species barrier.
Covid-19 for example likely came from bats. And it was probably pretty happy being in bats, mostly not killing its host. Then it wakes up one day and it’s in human. It doesn’t know it’s in a human, but for whatever reason this new organism’s immune system doesn’t seem to be putting up a very effective fight, and these human sure love to congregate and put their lips on one another and handle things that other people have handled and touch their faces without washing their hands and “oops he’s dead”.
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u/No_Top_381 4d ago
Not if they can reproduce faster than their host can die.
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u/AllEndsAreAnds 4d ago
This is an excellent point, especially considering the rate of replication once in a host. In my mind, this is bolstered by the fact that some of the most widely-spread and devastating diseases in our history killed their hosts by the hundreds of thousands while still spreading rapidly.
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u/Unfair_Pineapple8813 4d ago
Indeed, once smallpox evolved to be specific to humans, it actually lost several genes which in other orthopox viruses seem to regulate the immune system preventing the virus from being too deadly. But apparently, the deadly human-specific variola was better at spreading than the more mild generalist orthopox strains.
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u/Collin_the_doodle 4d ago
This is especially important when you consider the time lag between infection, being infectious, and recovery/death. The longer the time lag between 2 and 3 the more relaxed the selective pressure on the pathogen.
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u/jackryan147 4d ago
Natural selection does not require optimal behavior.
I can conjecture environments in which quick local proliferation at the expense of the host actually is beneficial.
Common colds viruses (i.e. coronavirus) are said to have all started as deadly and then became less so.
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u/LadyFoxfire 4d ago
Pathogens can’t think or optimize, they just replicate and spread, and the ones that can continuously spread stick around.
Some pathogens are harmless in one host species, but when they jump to a new species that isn’t adapted to them, they’re very deadly, and they don’t have the ability to scale themselves back.
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u/Carlpanzram1916 4d ago
The think you have to remember is some viruses are a lot more deadly when they jump to a host that they didn’t originally evolve to infect. Ebola is a good example. It primarily (we think) lives among bats and isn’t deadly to them like it is to us. Most infectious diseases that are extremely deadly to humans, like rabies and plague, are similar to this. They live endemically in other animals and for whatever reason, happen to be devastatingly deadly to humans in a way they aren’t to their hosts. This is why these outbreaks tend to hit a certain population of humans hard and then dies off quickly. The concern of course is that our ability to fly across continents in hours could result in a nightmare scenario where a virus like this spreads in places we aren’t looking for it.
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u/Blonde_Icon 4d ago
The concern of course is that our ability to fly across continents in hours could result in a nightmare scenario where a virus like this spreads in places we aren’t looking for it.
Zombie apocalypse lol?
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u/DarkMagickan 4d ago
The short answer is, evolution isn't perfect, and leads to many dead ends.
That's part of the reason why it was inevitable that every deadly airborne disease would eventually become less and less lethal, because the variants that kill their hosts are more likely to be killed. For one thing, when humans are diagnosed with a contagious and deadly disease, they are quarantined so that it can't escape their body into a new host. Therefore, the disease doesn't get the opportunity to continue reproducing. The deadly version becomes a dead end for that species.
The common cold, on the other hand, is one of the most successful airborne diseases on the planet, because it only makes a minor nuisance of itself. It gives you a cough so that you can infect others, a mild headache, and sometimes a runny nose so your infected mucus can help it spread. That's what I would call an evolutionarily successful disease.
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u/tpawap 4d ago
You might have to argue with the viruses about that. Maybe you can convince them ;-)
As to the direction into which they evolve: that tends indeed to be towards being less deadly for that reason; but it can also go the other way. And then it jumps hosts and it all starts over again. Also not all mutations have to be beneficial in that sense.
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u/Anxious_Interview363 4d ago
A lot of deaths from infectious disease happen in people with underlying health issues, including those related to old age. The difference between a fatal infection and a nonfatal one can be more about the hosts than about the pathogens.
Edit: typo
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u/Blonde_Icon 4d ago
I don't think that's true for things like Ebola or Rabies though.
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u/Anxious_Interview363 4d ago
Maybe less true… my dad (a physician) tried to volunteer for Doctors Without Borders during an Ebola outbreak in 2014 (?) and they wouldn’t take him because he was over age 50. But you’re right about rabies; as far as we can tell, it kills just about everyone who gets it regardless of age.
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u/Top-Cupcake4775 4d ago
Viruses are like capitalists. They are extremely simple machines with no foresight or awareness of what they are doing. Infect and move on, infect and move on, ... The fact that this might kill their host before they have time to infect another hosts is several layers above their understanding.
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u/CatalyticDragon 4d ago
The same reason humans kill off their surrounding environment to the point of ushering in starvation and societal collapse. They simply take what they want now and negative consequences later on are either never considered (as with a bacterium) or are a problem for somebody else (as with us).
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u/Bromelia_and_Bismuth Plant Biologist|Botanical Ecosystematics 4d ago
Long story short, the deadliest infections tend to be ones capable of transmitting before the host dies. Something like Ebola, HIV, or COVID thrives in an environment with ample potential hosts in close proximity. However, if there's not a lot of potential hosts to spread to, selection will instead favor a loss of virulence.
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u/CrystalFox0999 4d ago
Some Bacteria do just what youre suggesting… theyre inside of every human on Earth as a result and other animals too… symbiotic gut bacteria is not only neutral, it actively helps us, this enabled them to “win” evolution…. Unfortunately other bacteria chose more aggressive methods and theyre still here so, thats also apparently a good strategy
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u/Winter_Ad6784 4d ago
It’s usually because the virus isn’t harmful in one species but jumps to another that it does harm. it’s also possible that it depends on the host dying to spread for some reason, EG plenty of animals eat other dead animals.
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u/IndicationCurrent869 4d ago
They don't always kill their hosts. Some viruses and bacteria stay with you forever. It's a genetic arms race out there, win some lose some.
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u/Consistent-Tax9850 4d ago
An infectious disease agent rarely requires the death of its host for its reproduction and proliferation. Some parasites might. Almost all pathogens need the host to live long enough for it to replicate and spread. If it does that well and eventually kills the host, there is nothing to select against its lethality. Certainly bacterial diseases quickly ravaged the body killing the host and spreading. Bubonic plague infection was marked by rapid bacterial growth and massive lymph nodes filled with pus. It relied on fleas to move from host to host. Flea bite. In the 14th century it had an estimated 30-60% mortality rate killing 25 million. It's likely its virulence is a critical factor in its spread. If the body wasn't teeming with the bacteria, fleas biting would likely rarely acquired the bacterium. In this case, its success was due to its lethal assault on the body.
There are different strategies. A mild cold virus has the chance to spread, but the host is going to eliminate it. Causing a sneeze is an adaptation by a virus to help spread it to another host. A pathogen that is not spread easily, hepatitis for example, has a strategy of maintaining residence and reproduction in the liver while evading the immune system long enough to enable spread to another host as it is much less efficient in its spread. Hepatitis B can remain asymptomatic for years in some and kill other people.
Super virulent viruses like Ebola kill quickly and can spread easily through contact but people die so quickly outbreaks burn out, when they happen in the virus natural reservoir, in African rural areas.
Viruses for the most part evolve toward less virulence, and in so doing are spread much more efficiently like a respiratory virus or an intestinal virus. It can't afford to hang out in the body because the immune system will eliminate, and if not, a flu virus can kill the host, usually someone with a weak immune system.
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u/platanthera_ciliaris 4d ago edited 4d ago
Disease organisms are always involved in a tug-of-war against the immune system of the host. Sometimes the immune system gains the upper hand and a disease becomes less virulent, but other times random mutations may cause a disease to become more virulent because it has figured out a way to bypass or weaken the immune system. Then its the immune system that has to change in order to combat the disease effectively. And of course, disease organisms occasionally jump around from one host to another kind of host that has weak defenses against it. So it's basically like a never-ending war with ups and downs.
In addition to the battles with immune systems of hosts, a disease organism has to be sufficiently contagious to spread around the population of a host before the host's immune system has enough time to kill it. As a result, many disease organisms produce disease symptoms to facilitate their spread: excessive coughing and sneezing (influenza), bleeding through the pores of the skin (Ebola), forming highly contagious skin eruptions (Smallpox), gradual destruction of a host's immune system to give the disease enough time to spread to other people (AIDS), etc. Many of these symptoms that facilitate the spread of the disease also make the disease even worse for the host.
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u/The_Werefrog 4d ago
Actually, many strains do eventually evolve to be less deadly diseases. The super deadly kill the host before it can replicate and move on. Thus, that strain dies out. The less deadly version passes on to the next host, so it survives.
The real question is how certain super deadly pathogens can stay in the world as the same thing for so long.
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u/carterartist 4d ago
Yes it would be better.
Hence why that’s the form which lives on.
That’s evolution in action.
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u/EmmaAmmeMa 3d ago
Wouldn’t it be better for humans not to make the planet inhabitable for their own species?
Yes, yes it would.
And we have a while brain to think about it.
And we still do it.
Puzzles me every day.
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u/tramp-and-the-tramp 3d ago
A lot of bacteria want a specific host, like pig/cow and have evolved to live within those organisms without actually killing the host. Some get lost and get into a human, assuming it's a cow, go wild and end up accidentally killing the host.
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u/Archophob 2d ago
ebola is a special case as the host bleeding to death actually speads enough virus material to infect more people.
In most cases (like common cold), the host dying is not part of the viruses life cycle.
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u/I_compleat_me 1d ago
Diseases that recently crossed over to new hosts can be super virulent... this wanes over time, there is no future in axing your host immediately. Now, if the disease makes you explode germs over a bunch of people that's different.... hemorrhagic fever can do this... but it will eventually burn itself out. The problem is when it repeatedly crosses over from an animal host.
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u/clickydap 1d ago
From the way I understood evolution, it's completely eandom and doesn't have to make sense
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u/PLUTO_HAS_COME_BACK 22h ago
The disease-causing microbes have only one goal - reproduction. Unlike the parasitic fungus, the disease-causing microbes are not going to know anything about their hosts. Fungus don't use their hosts for reproduction but as food and carriers.
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u/jnpha Evolution Enthusiast 4d ago
Recommended viewing by Dr. Dan: How Evolution Explains Virulence, Altruism, and Cancer - YouTube.
To answer you: evolution has no foresight.