Question: viruses (the organic kind)

[Gord]

In summary: why do some viruses (Ebola, AIDS, etc.) kill their hosts? Isn't this detrimental to their ability to survive/reproduce/etc?

I'm probably missing the obvious reasons...

 

[Cogman]

As I remember According to the definition of life, Viruses are not alive. Next, Viruses sole purpose is basicly to reproduce, nothing more.

 

[eigen]

Originally posted by: Cogman
As I remember According to the definition of life, Viruses are not alive. Next, Viruses sole purpose is basicly to reproduce, nothing more.

Some say yes some say no

http://www.tulane.edu/~dmsande...ryfavwebfaq.html#alive

http://en.wikipedia.org/wiki/V...non-living_or_alive.3F

I say it doesnt matter they do some form of information transfer. They provide some of the mutagenic capacties needed for evolution to take place...So cut them some slack.

 

[gururu]

viruses are not considered to be 'living' by most virologists. only cells are alive.
for most viruses, its either kill or be killed. Ebola for example, would get wiped out by the immune system if it didn't proliferate so fast.
Other viruses like herpesviruses reach a nice balance with host immunity and are generally benign.

 

[jagec]

Originally posted by: Gord
In summary: why do some viruses (Ebola, AIDS, etc.) kill their hosts? Isn't this detrimental to their ability to survive/reproduce/etc?

I'm probably missing the obvious reasons...

you're right, those viruses ARE considered "less successful" due to the way they kill the host.

 

[Paratus]

What interesting is that as the viruses reproduce and go through different generations they inevetiably evolve into less lethal forms. Look at the Flu - killed 20 million in the early 20th century, now its only dangerous the very old or young. Both humans and virii adapt to each other.

 

[gururu]

Originally posted by: Paratus
What interesting is that as the viruses reproduce and go through different generations they inevetiably evolve into less lethal forms. Look at the Flu - killed 20 million in the early 20th century, now its only dangerous the very old or young. Both humans and virii adapt to each other.

another thing to consider is that viruses have to compete with each other for cellular control. In evolving to compete, they may actually become more lethal. It's a lot of work to be a virus. we really have to figure out how to put them out of their misery.

 

[rgwalt]

Originally posted by: Paratus
What interesting is that as the viruses reproduce and go through different generations they inevetiably evolve into less lethal forms. Look at the Flu - killed 20 million in the early 20th century, now its only dangerous the very old or young. Both humans and virii adapt to each other.

I think that has more to do with better medical technology than anything else.

R

 

[fuzzynavel]

I think that has more to do with better medical technology than anything else.

I think you are right......look at the plague of the 16th century......killed 10's of millions across europe....now it is about as effective as the common cold if it is treated correctly!!!

From what I remember of molecular Biology....Viruses destroy cells from the start......they infiltrate the cell and then are reproduced inside the cell till it bursts...they then move on to other cells and produce exponentially...taking over cell after cell after cell.....if your body or the drugs can't cope then you are dead....

Virii do evolve as well.....

so do bacteria......look at the flu and MRSA......they were pretty well controlled by antibiotics...but then some people think....I'm all better I'll just stop taking my medicine......this is where the virii and bacteria win.....that person won't necessarily be reinfected...but that bacteria or virus inside will adjust and the same antibiotic will be less effective next time until the viru/bacteria is totally immune to that antibiotic....that is why you are told to take the full course of antibiotics.......

 

[Mark R]

Originally posted by: rgwalt

Originally posted by: Paratus
What interesting is that as the viruses reproduce and go through different generations they inevetiably evolve into less lethal forms. Look at the Flu - killed 20 million in the early 20th century, now its only dangerous the very old or young. Both humans and virii adapt to each other.

I think that has more to do with better medical technology than anything else.

Actually, it probably isn't completely.

Prior to the major 1918 global pandemic, 'flu was a relatively minor illness. Yes, it was dangerous to the infirm, elderly and young, but death was unsual (estimated at probably less than 1 in 1000).

Something was different about the 1918 virus. It was much more contagious and much more virulent. It would rapidly infect young fit people, and they would develop pneumonia and die (death rate probably 1 in 40).

This was probably not so much due to a significant change in the virulence of the virus, but instead because it had 'changed its spots'. Flu viruses endemic in a population don't generally cause much illness because the population is immune. Slow changes in the virus mean that the population can retain some immunity against slightly different viruses. The big pandemics are thought to have occurred because the virus simply picked up a new coat from another virus (e.g. from swine flu or bird flu), to which the population have no immunity at all.

Finally, it's not always that case that pathogens continually evolve into less lethal forms - it depends on the mode of operation of the pathogen and the populations. The classical example is HIV. HIV has a very rapid rate of genetic change, and can evolve rapidly. The HIV infecting the general population in Africa is considerably less virulent than believed to be at discovery. Conversely the HIV infecting drug abusers and gay men in Western inner cities, is extremely virulent and causes rapid progression to AIDS.

The difference is in the methods of spread. In Africa, spread is sexual primarily sexual, and as many relationships are stable and monogamous, a virus that replicates quickly inside its host will fail to spread. Conversely, drug abusers may share needles with a huge number of people over a period of months, so an aggressive virus which rapidly overwhelms its host does not lose its transmission ability.

I think you are right......look at the plague of the 16th century......killed 10's of millions across europe

Actually, this disease (plague) is a highly dangerous disease today, although it can be treated with penicillin. The bug that causes it (Yersinia pestis) is highly virulent and can cause very severe pneumonia as well as causing septicaemia. Thankfully, better hygeine and removal of its vector, the flea, and its host, the black rat, has greatly reduced the incidence of this disease.

 

[fuzzynavel]

Nice informative post Mark R!!

look at the plague of the 16th century......killed 10's of millions across europe....now it is about as effective as the common cold if it is treated correctly!!!

from my post earlier

I didn't say it wasn't dangerous.....I said it was easily treatable if infected!!!

 

[Mark R]

I didn't say it wasn't dangerous.....I said it was easily treatable if infected!!!

Point taken, but even with correct treatment it still carries a risk of death of about 1 in 10 to 1 in 5. It is certainly a far more effective pathogen than the common cold.

 

[gururu]

with respect to the flu, its recently been shown that the 1918 epidemic was caused by a flu strain carrying a unique gene which increased its ability to infect many more cell types than the strains we commonly deal with today; indeed it was much more virulent. If they took that gene and put it into a 'normal' flu strain, the new strain could induce the same symptoms in mice as the 1918 strain did in people. This was just reported about a month ago by Kawaoka.

lastly, medicine has had very little effect on the evolution of viruses. There is not a single cure for any viral disease yet. Vaccines still remain the best solution, but these as well are not cures. The immune system is still the best thing we got. Unfortunately, some viruses have taken to infecting immune cells. Others have learned to evade it to a degree.

as I've mentioned before, viruses evolve in the face of two factors:

1) Evasion of immunity
2) Competition with its self

Evasion of immunity can lead to increased lethality, but it can also lead to persistence or latency. Competition with self can lead to the selection of more virulent or lethal forms. Any mutant that propagates 100 fold over an unmutated form is going to spread to the next host faster, even if it means the host dies. In this case, the virus is taking advantage of the fact that animals are social creatures. Increased virulence leads to the types of epidemics we've seen with the 1918 flu. Similarly the bacterial pathogen responsble for the black plague spread very fast and did so by replicating so fast regardless of killing host rapidly.

 

[gururu]

Originally posted by: Mark R

The difference is in the methods of spread. In Africa, spread is sexual primarily sexual, and as many relationships are stable and monogamous, a virus that replicates quickly inside its host will fail to spread. Conversely, drug abusers may share needles with a huge number of people over a period of months, so an aggressive virus which rapidly overwhelms its host does not lose its transmission ability.
.

ok this is not accurate. the reason HIV is spreading so fast in certain regions of Africa is because infected men are having sex with multiple women, particularly very young women, who are then in a position to spread the virus to other men. as an example, their was just a report that in a certain province, tens of thousands of women had been raped, so in many if not most cases, its not even consensual. Strains of HIV that people got in the 70's-90's were mostly able to be 'repressed' through antivirals, regardless of where or how they got the virus. Forms gotten by drug users and gay men were no more virulent than strains in Africa. In fact, HIV-related AIDS kills more people in Africa by several fold than HIV-related AIDS does here.
In the last ten years, there's been an influx of new strains that are slightly more resistant to antivirals, but that has nothing to do with the conditions of transmission. It is simply that we are selecting for mutants with an increased tolerance to drugs.

 

[Jeff7]

1) Evasion of immunity
2) Competition with its self

Evasion of immunity can lead to increased lethality, but it can also lead to persistence or latency. Competition with self can lead to the selection of more virulent or lethal forms. Any mutant that propagates 100 fold over an unmutated form is going to spread to the next host faster, even if it means the host dies. In this case, the virus is taking advantage of the fact that animals are social creatures. Increased virulence leads to the types of epidemics we've seen with the 1918 flu. Similarly the bacterial pathogen responsble for the black plague spread very fast and did so by replicating so fast regardless of killing host rapidly.

There will be freaks of nature, like the poison arrow frogs and things of that sort - unbelievably toxic. Definitely more poisonous than needed. If the poison will kill one predator, why make it toxic enough to kill hundreds of them? It just sort of happened that way, and there wasn't any reason for it to evolve away.

Viruses - the ones that are able to spread will survive. Those that either mutate in a way that prevents spreading - whether it be from an inability to become airborne, or from killing hosts too quickly, or even infecting the "wrong" kind of host - simply die off. Such is the way of evolution. We don't see many of the "failed" species because of simply that - they've failed. The mutations did not produce viable life forms, and thus the life form dies.

 

[gururu]

Originally posted by: Jeff7

There will be freaks of nature, like the poison arrow frogs and things of that sort - unbelievably toxic. Definitely more poisonous than needed. If the poison will kill one predator, why make it toxic enough to kill hundreds of them? It just sort of happened that way, and there wasn't any reason for it to evolve away.

Viruses - the ones that are able to spread will survive. Those that either mutate in a way that prevents spreading - whether it be from an inability to become airborne, or from killing hosts too quickly, or even infecting the "wrong" kind of host - simply die off. Such is the way of evolution. We don't see many of the "failed" species because of simply that - they've failed. The mutations did not produce viable life forms, and thus the life form dies.

Nature rarely does anything that is not needed. Obviously there is something about the amount of poison that these frogs have that made them successful. To comment on whether they produce enough, too much, or not enough poison is IMO presumptuous.

So I'm a virologist, and it indeed is a great mystery in the field. What is the advantage in killing host? What you'll see more often than not is that DNA viruses do a much better job at getting into hosts and staying on the long term. RNA viruses on the other hand tend to be very 'ambitious'. Cells have evolved many mechanisms to fend off RNA viruses, because they use a lot of replication intermediates that are unique and dissimilar to cellular molecules. Cells have developed a number of mechanisms to identify and destroy these replication intermediates. A slow growing RNA virus won't have a chance. DNA viruses on the other hand, for the most part, don't use these replication intermediates and go by unnoticed. There are still mechanisms by which they are detected, but they are not as efficient. This is one example of why a virus (an RNA one) will try to get the most bang for the buck. They can't afford to wait.
We do consider viruses that maintain persistence as successful viruses. We also consider viruses such as ebola and HIV successful as well. variants of these viruses have been around for at least thousands of years and they are not soon on their way out.

 

[Gibsons]

To follow up somewhat on what gururu said, it's worht mentioning that we're mostly talking about virii that are or can be lethal.

Arguably, the most successful virii are the ones we don't talk about much. Epstein-Barr Virus for instance causes mono ("kissing disease") and is associated with a few rare cancers (very strong assoiciation, but the cancers are still rare), but something like um... 95% of the US population carries it (number subject to error from my bad memory). For the most part it's benign, typically people get infected as children and show some cold-like symptoms briefly, then carry the virus for rest of their lives with no symptoms (for some reason it's a lot rougher when it first infects an adult). One might argue that this makes EBV and similar virii more "successful" than other more lethal ones.

Then you have latent retroviruses, the tons of virii that seem to little more than cause common colds, herpes, etc.

 

[Jeff7]

Nature rarely does anything that is not needed. Obviously there is something about the amount of poison that these frogs have that made them successful. To comment on whether they produce enough, too much, or not enough poison is IMO presumptuous.

Good point - maybe then it's just that it's not needed anymore, but hasn't had any reason to evolve away. For example, we still have appendixes. Some snakes still have small rear limbs. These things aren't needed anymore, but they're still there. They just haven't become "expensive" enough, in terms of metabolism, to have reason to evolve away. Same with those poison arrow frogs. Ok, maybe their distant ancestors were eaten by huge creatures, and they needed that really toxic poison. But now there's nothing that size around to eat them, just small birds and snakes, but they've still got the potent poison.

 

[Taejin]

Just a quick piece of information - if my own limited knowledge is correct, poison arrow tree frogs aren't poisonous naturally. I believe it's something that they consume in their diet that they metabolize into poison. If they don't feed on it, they're harmless as a ..rabbit..or another frog..

 

[Pulsar]

The "flu" is actually a strain of virus that comes from avians (birds). The pandemic that occurred in 1918 was when a new flu strain made a species jump. Human immune systems didn't "see" the invader because they hadn't seen it in the past.

Several species of animals, most notably pigs, are susceptible to both human and avian influenza. It's believed that the 1918 pandemic started in Kansas, possibly from a bird - pig - human jump.

Even now, new flu strains are making small jumps to humans. SARS was terrifying to scientists because they initially believed it was a strain of influenza that had jumped.

In addition, repeaded contact between the virus and humans can cause it to mutate enough to jump. In each outbreak in recent years, hundreds of thousands of chickens and other birds have been destroyed in the fear that some may have been exposed to recent strains that are jumping.

Most experts believe that we are long overdue for another large scale jump. Anti-viral agents CAN slow the flu and mitigate some of its affects, but they are being held in the hopes that if they are ever needed, they will prolong the outbreak long enough for a successful vaccine to be developed.

 

[PumpActionWalrus]

AIDS doesnt actually kill the host, it festroys the immune system, it lets something else kill the host

 

[Matthew Daws]

I think the confusion arises because we are putting a rather human spin on what "sucessful" means.

Suppose, hypothetically, that a single mutation of a virus causes the virus to both become 100% deadly, but also allows it to become increadibly good at infecting and spreading. This is a clear improvement, so this form the virus will repoduce and come to dominate the other form the virus. The virus will then spread, infect the entire population of its host, and then obviously die out. Now, from a human, intelligent, perspective, we can say that the newer virus is not "sucessful" in the long term, as it kills all its hosts. However, that's not how evolution works: it is simply better at reproducing, and so comes to dominate.

More practically, I believe that Ebola lives in monkeys (not sure about this though) and isn't deadly to them. It's a slight freak of luck that it happens to be deadly in humans.

--Matt

 

[imhotepmp]

Originally posted by: fuzzynavel

I think that has more to do with better medical technology than anything else.

I think you are right......look at the plague of the 16th century......killed 10's of millions across europe....now it is about as effective as the common cold if it is treated correctly!!!

From what I remember of molecular Biology....Viruses destroy cells from the start......they infiltrate the cell and then are reproduced inside the cell till it bursts...they then move on to other cells and produce exponentially...taking over cell after cell after cell.....if your body or the drugs can't cope then you are dead....

Virii do evolve as well.....

so do bacteria......look at the flu and MRSA......they were pretty well controlled by antibiotics...but then some people think....I'm all better I'll just stop taking my medicine......this is where the virii and bacteria win.....that person won't necessarily be reinfected...but that bacteria or virus inside will adjust and the same antibiotic will be less effective next time until the viru/bacteria is totally immune to that antibiotic....that is why you are told to take the full course of antibiotics.......

Not only that they can actually stay dormant for many years just like AIDS herpes, etc.

Some virii have actually evolved mechanisms that induce a high mutation rate.

High mutation rate= fast evolution= survival.

thats the name of the game.

Imouthes

 

[Farvacola]

No, Viruses are alive, but they are not classified as organisms. The only requirement they do not meet is the maintaining of homeostasis. In other words, the killing of their host doesn't go agains their nature because they do not follow the same rules organisms follow.

 

[gururu]

Originally posted by: Farvacola
No, Viruses are alive, but they are not classified as organisms. The only requirement they do not meet is the maintaining of homeostasis. In other words, the killing of their host doesn't go agains their nature because they do not follow the same rules organisms follow.

you do realize that there is a nobel prize available to the person that proves a virus is alive.