Why do people die from viruses?
April 4, 2013 5:03 PM   Subscribe

How can I explain to my 7-year-old why viruses kill their hosts?

My daughter is thinking about viruses and why we sneeze and cough and how these things are vectors for virus reproduction. She asked me yesterday why viruses would kill their host since this would seriously limit the virus's ability to spread itself around. I didn't have a good answer for this, and the answers I've found on the internet don't really break it down to first grade level. Any help is much appreciated.
posted by stinker to Health & Fitness (18 answers total) 3 users marked this as a favorite
 
Simplistically, you could say that viruses aren't perfect -- sometimes they overshoot and then kill their hosts too quickly. That virulent a virus doesn't get very far. The most successful viruses at killing are those that allow their hosts to live long enough to be passed around. The ones that get spread the most are those that never kill their host.

This personifies the virus, which isn't quite correct, but I think it's allowable for first graders.
posted by Tandem Affinity at 5:11 PM on April 4, 2013 [2 favorites]


Viruses kill their hosts as a side effect. They only "want" to reproduce, so they move into a living creature and subvert some of its cells to making viruses instead of doing their normal jobs. The virus has no malice against the organism.
posted by zadcat at 5:12 PM on April 4, 2013 [1 favorite]


Viruses don't exist to kill, they exist to spread. Whether or not they kill the host after spreading to new hosts is irrelevant in terms of the evolution of the virus. The most "successful" viruses (evolutionarily speaking) are those that spread to the most number of people, and they do that by not killing their hosts too quickly. It doesn't matter if they do kill them or not, just that if they take a long time, the host can spread the germs around more. The opposite of this is a virus that kills its host too quickly, and limits the amount that it can spread (I think ebola is an example of this).

So your daughter is right - killing the host is bad for the virus, but the virus can evolve to mitigate that by having a longer incubation period or evolving symptoms that spread more germs (runny nose, sneeze, diarrhoea, vomiting).
posted by EndsOfInvention at 5:12 PM on April 4, 2013


This may (or may not) help, but it's uber cool no matter what.
posted by hamandcheese at 5:16 PM on April 4, 2013 [2 favorites]


Response by poster: I think she understands that viruses aren't trying to kill their hosts, she just wonders how it would happen that one would. Are these evolutionarily "young" virus that just haven't evolved to be less lethal yet?
posted by stinker at 5:16 PM on April 4, 2013


http://en.wikipedia.org/wiki/Optimal_virulence

This is a primer on the idea i think you're looking for.
posted by skewed at 5:25 PM on April 4, 2013 [3 favorites]


Some people are healthier than others. People who are healthy have a better chance of surviving viruses. Sometimes, someone can be so healthy that a virus, such as a cold, doesn't even make them feel all that bad.
People who are already not feeling good sometimes cannot survive a virus. That is why it is important to eat healthy and take good care of ourselves and never ever smoke, because we want to keep our bodies strong and healthy.
posted by myselfasme at 5:26 PM on April 4, 2013


Well, take the flu. There are a non-trivial number of deaths each year, but those deaths mostly occur in the very young and the very old, i.e. those who have weaker immune systems and organ function in general, so if the virus is optimized to balance between making lots more viruses and keeping its host alive, that corresponds to a certain strain on the body. In the majority of the population, this works quite well, but under that same strain, the young and old will die.

Of course, HIV can also kill its host. For HIV, my guess would be that it can be relatively quiet in your immune system for so long that the number of new hosts it would gain from increasing the incubation period is relatively low.

On preview, what skewed said.
posted by Maecenas at 5:27 PM on April 4, 2013


Are these evolutionarily "young" virus that just haven't evolved to be less lethal yet?

A lot of viruses that cause diseases in humans are the result of an odd mutation in a virus strain normally carried by some kind of animal (most types of the flu do this, as did HIV), where the mutation allows it to be infectious to humans to whatever degree. So, they've generally done perfectly well from a natural selection standpoint until then and the mutation just gives them broader range of potential hosts.
posted by LionIndex at 5:30 PM on April 4, 2013 [1 favorite]


Virulence is all well and good but generally, the cytokines from your own immune system overreacting to the virus is what actually kills you, with stuff like the flu. Your symptoms from most viruses are due to your immune system reacting to the infection, not the virus itself.

Ebola, on the other hand, is a little more complicated.
posted by hobo gitano de queretaro at 5:32 PM on April 4, 2013 [1 favorite]


But then there are examples like the 1918 Flu outbreak, where it was the primarily the young fit who succumbed from the cytokine storm- in other words, the response from the immune system was so intense that the body could not survive it. Those with weaker immune systems tended to survive.
posted by ambrosia at 5:32 PM on April 4, 2013 [1 favorite]


"I think she understands that viruses aren't trying to kill their hosts, she just wonders how it would happen that one would. Are these evolutionarily "young" virus that just haven't evolved to be less lethal yet?"

Sometimes, like in the case of things like the Nipah virus, but not always. While my Secret Universe post linked above is cool, this is really the post you're looking for.

Those two papers demonstrate empirically that even extreme virulence will indeed be selected for if a virus can get enough benefit for transmissibility out of actions that also harm their host. Broken down simply, if by growing in you so fast it kills you a virus manages to spread enough virions to infect two more people on average it still wins even though you're dead and it can no longer use you as a host. Needless to say this is not a dynamic that works out so well for us when it works out for viruses.
posted by Blasdelb at 7:13 PM on April 4, 2013 [3 favorites]


Symbionts, one of at least two organisms living in each other’s presence in a metabolically intimate way, often will care little for their hosts and make as many infectious particles as possible at the host's expense, thus increasing virulence, or how much the parasite messes up its host as a consequence of infection. In this strategy it doesn't matter so much that the host becomes quickly unsuitable because the parasite has already found replacement hosts sneezed on, or transmitted to, by the time that happens. Or it can do the opposite and try its best to reduce impact on the host, shed infectious particles slowly or even not at all, and thus not need to spread to quickly because it will last a while in each host. Commensal symbionts are at that end of the spectrum, and have become so adept at not messing up their host as to actually benefit them in some way. On the other end are parasitoids. These are the parasites that not only mess up their host in their race to infect as many more hosts as possible, but spend the majority of their life cycle doing so and ultimately sterilizes, kills, or sometimes consumes the host in the process. Viruses, well at least Eukaryotic viruses, are stuck towards this end of the spectrum where they can't possibly provide any benefit to their hosts and the most helpful state they can accomplish is mostly harmless. While moving towards the mostly harmless end of the spectrum available to viruses has its benefits, as your daughter has already reasoned out, it also has its drawbacks - well for the virus anyway - as it effectively prevents viruses from replicating rapidly, existing in high titers at all, or doing a whole bunch of other things that would help the virus spread from host to host.
posted by Blasdelb at 7:29 PM on April 4, 2013


I think it's probably important to express that viruses don't 'try' to do anything. They spread. If they're contagious enough, the life or death of the host is irrelevant to the spread. Obviously a virus that's massively fatal is only likely to go around one time, though.

But yes, the ones that hang around year after year after year tend not to kill their hosts, or at least not very quickly.
posted by empath at 7:32 PM on April 4, 2013


Hobo gitano de queretaro makes a good point that a clever, curious 7-year old might be interested in knowing.

Also, by analogy, there is the question of why humans, which are so much more complicated than viruses, can, nevertheless poison and damage their own environment to an astounding degree. Or yeast, which will devour sugar and produce ethanol until there is too much of it for them to keep going. Or the earliest photosynthetic organisms, which dumped so much waste oxygen into the air that the atmosphere went from being a reducing environment to an oxidizing environment.

A related concept is carrying capacity.

Viruses are particularly fascinating though, because of their bare-bones simplicity.
posted by Good Brain at 7:33 PM on April 4, 2013


Are these evolutionarily "young" virus that just haven't evolved to be less lethal yet?

You could think of it that way; SIV (HIV for simians) is much less deadly for simians than HIV is for humans. Also, it is thought (not without controversy) that syphilis was native to the Americas and spread to Europe after 1492. The accounts of the virus in Europeans from that time suggest it was lethal and debilitating very quickly, unlike today. This suggests that some strain of syphilis became less virulent and became more successful in a population that had no natural immunity.

That's not to say it's inevitable with every virus. Ones that are very good a spreading really don't need you to live very long, so less deadly strains may not realize a particularly sizable advantage.
posted by spaltavian at 7:56 PM on April 4, 2013


Viruses play the Game of Life. The object of the Game is to Keep Playing as long and as often as possible. If they Lose and go extinct, it's Game Over and they don't get to play again.

Sometimes they do just better enough than the other players that the viruses get to Keep Playing, even if the other players don't do as well and come down with a cold, or whatever.

Sometimes, some viruses get so good at the Game of Life that they make all the other players Lose, and these viruses end up taking themselves out of the Game in the process, because there is no one left to play the Game with.

Playing the Game of Life is always a delicate balancing act, because all the viruses are not only playing against people, they are also playing against other viruses and bacteria, and there's always pressure on one virus to be as good as possible at playing the Game, even if that might mean it occasionally risks accidentally making the other players Lose.
posted by Blazecock Pileon at 12:29 AM on April 5, 2013


The ones that really tend to kill people quickly are often viruses whose primary "target" is a different species -- take swine flu (swine), or the terrible Ebola family (which comes from apes) -- so it's adapted to transmission among that species with minimal lethality, and presumably would have to adapt if it were to persist in ours (but Ebola would just wipe a lot of us out before it had a chance). Another example is that hantavirus that showed up a while ago -- turns out it lives mostly in rodents, which is why several Native American tribes had traditional strictures about burning a blanket after a mouse runs across it, etc. So it only kills us when it gets across the species barrier now and then.
posted by acm at 7:19 AM on April 5, 2013 [1 favorite]


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