why do viruses try to kill the thing keeping them alive?
In: Biology
They mostly don’t. A successful virus is gentle to its host. Something like the cold or flu or herpes. Viruses that are often fatal usually don’t last long and it’s for exactly the reason you’ve noticed.
You are making an assumption of intent with the word kill. Viruses do not even meet the basic definition of life. Thus they cannot have intent.
Viruses basically are a package of DNA or RNA with the tools to infect cell and replicate. That is literally thier only function. Death of the host life form is merely a side effect of this. At its heart it is not that different than the trillions of microgoranismed you destory everyday by walking around. You do not have intent to kill them it is just a matter of fact of your daily life processes.
Viruses are usually most deadly immediately after they have jumped to a new species. They will then tend to evolve to become less virulent, because as you say they want to keep you alive to spread.
We’ve seen many examples of this happening, the HIV virus has become slower to cause AIDS which combined with newer treatments has made it much less deadly. Even early variants of COVID-19 were less deadly than later variants like Omicron (although it still remained more deadly than the flu virus).
To add on to u/TheJeeronian ‘s correct answer, if you think about it, most lethal viruses *weren’t originally human viruses*.
Smallpox came from rodents. HIV came from chimpanzees. Covid 19 came from some animals too.
The lethality is because the virus is evolved to be essentially a cold *in some other animal*. But humans aren’t the same as those other animals, so a virus’ “attempt” to give another animal a cold is enough to kill a human.
Eventually, after the virus stays with humans long enough, the trend is for the virus to become less lethal since lethal ones kill their hosts and then themselves. We saw this recently with Covid variants becoming less lethal over time.
It is not possible for a virus to reproduce without disrupting the function of the organism they are replicating in. In theory, the perfect virus would replicate without its host being harmed or even noticing. But the fact that your cells are using energy producing proteins they were not meant to produce and filling up your body with them means harm will probably happen, and the faster the virus replicates the more they will tend to disrupt the body unless they evolve specific mechanisms to avoid this disruption.
So there’s basically a trade-off. Viruses will evolve to reproduce as fast as they can get away with, without harming their host’s ability to spread them to new hosts. If they reproduce quickly and a few of the hosts die from the overwhelming number of viruses, but they spread the virus to many other hosts before they die? That is a success. If they reproduce quietly and the host doesn’t notice them and is able to walk around and spread the virus to other people for a long time? That is also a success. There are many strategies that work.
Viruses aren’t trying to do anything. It’s just a bit of code that plugs into a cell and tells the cell to make more of the code.
Overall a lot of viruses don’t kill their human hosts, or at least they are able to be able to reproduce and spread to other people before they have a chance to either be eradicated or kill their host.
Take HIV for example, it can be contagious for many years, sometimes unnoticed, before it actually does you in indirectly. Rabies in particular isn’t exactly meant for humans as bats are a much better host, and generally less intelligent and more toothy animals like canines also make better hosts for transmission via bites. Weaker viruses like Rhinoviruses(Cold), Norovirus, Coronaviruses(Cold) or Influenza(Flu) go straight for being highly contagious and highly prone to mutating since they tend to be evicted from the body very fast. Herpes or Chickenpox is rarely of lethal concern.
Meanwhile, if you look at super lethal types of viruses. Smallpox was wiped out. Polio was nearly wiped out. Ebola is extremely rare despite potential for causing more widespread illness. The more dangerous strains of Covid were nearly wiped out as less dangerous mutations have replaced them. Many dangerous viruses, even if highly infectious, tend to be less successful since besides mortality, people are afraid of them enough to put more effort into quarantine and prevention.
They don’t “try” anything, they’re not sentient. Also, much of the damage done from sickness is actually a result of your body fighting the infections. Think of it like this — your white blood cells essentially carpet bomb any area with signs of infection, meaning that your own cells are collateral damage when your body is trying to fight something off that can kill you.
The immune system is way more sophisticated than that but it is a factor. Also, viruses fill your cells with more viruses when they propagate until your cells physically explode, resulting in a lot of waste material being left behind and of course dead tissue.
But there isn’t anything intelligent about viruses, how they target the body, and the mechanics behind how they propagate. It’s a series of chemical reactions between proteins and cell walls (hence why some animals are immune to some diseases or can host diseases while being asymptomatic), millions of years of adaptation, etc.
Any mutations in nature (which is basically any feature that an organism has) we see currently because it was successful by virtue of coincidence, finding success in some population of biodiversity where attributes it has allow it to succeed. It’s not like fish developed fins to swim, it’s that things with fins found success in bodies of water and therefore had successful offspring, same as viruses.
Virologist here just adding some clarification on some of the comments. Abnormally virulent viruses, like many of the ones that come from other animals and haven’t evolved to humans, will tend to evolve towards lower virulence because they’re causing too much harm to their hosts to transmit effectively or stably. BUT the opposite can also be true. Viruses need to exploit resources to facilitate their growth and cause symptoms (e.g., coughs) to promote transmission. Also, when viruses are competing with each other for hosts (as they often do in nature), generally the one that exploits its host faster and transmits before the host is incapacitated wins.
So viruses can evolve to be nicer to their hosts (see “avirulence hypothesis” – which is outdated but is still commonly taught to medical students and appears in a lot of these comments), or can evolve to be worse to their hosts. The balance between these two is the basis of the “virulence-transmission trade-off hypothesis”, which is the currently accepted theory of virulence evolution.
As an extra: while we often think that viruses that come to humans from other animals are more virulent, it’s entirely possible this is just detection bias. There could be constant zoonoses occurring that are sub-clinical that we don’t detect. It’s an ongoing area of research and our opinions are often coloured by decades of medical research that (quite rightly) focused its attention only on the worst of the worst of new diseases.
The virus doesnt have a brain, it doesnt intend anything, it just does.
Its just a randomly mashed together piece of biological code that happend to be able to propagate itself by highjacking cells.
But the most deadly viruses generally dont live very long since they indeed off themselves along with host. Its the viruses that harms us the least that has the best chance at survival.
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