Is there such a thing as an 'invisible' virus?
December 14, 2007 11:58 PM   Subscribe

Is there such a thing as an 'invisible' virus - have biologists stumbled across any 'diseases' that have no noticeable symptoms?
posted by unmake to Science & Nature (20 answers total) 5 users marked this as a favorite
About 8% of the human genome is thought to be ancient, foriegn DNA snippets that have merged with whatever was on its way to being modern human DNA, in our evolutionary chain. Here's the story of one such snippet that has been riding around with us for 4 million years.

Perhaps it once caused active disease, but perhaps, it didn't. But it doesn't seem to now, have become "unexpressed" as it rode around with us.
posted by paulsc at 12:10 AM on December 15, 2007

Short answer yes: ~11% of the mammalian genome is composed of retrovirus-like retrotransposons: "transposable elements in which transposition involves a process of reverse transcription with an RNA intermediate similar to that of a retrovirus". Compare this with only ~2.5% of the human genome which encodes unique (non-repeated) genes!.

Long answer: a parasitic life form evolutionairy speaking "wants to" coesist harmlessly with it's host. The long term process of evolution will naturally tend to sending the parasite and/or host extiinct, or to them coesisting peacefully.
posted by singingfish at 12:12 AM on December 15, 2007

I believe it is intrinsic to the definition of "disease" that there be impairment of body function, i.e., symptoms. So the presence of parasitic retroviruses can't really be accurately classified as a disease, I wouldn't think.
posted by Rumple at 12:19 AM on December 15, 2007

That is, parasitic retroviruses which don't cause any symptoms.

A similar observation can be made about your cells' mitochondria, that they are bacterial symbiots. Just try living without them!
posted by Rumple at 12:24 AM on December 15, 2007

There are plenty of diseases whose infectious agents don't always cause any symptoms, that is, there are people who are carriers but who don't get sick. There are some viruses which frequently don't cause symptoms, like cytomegalovirus. I think it's reasonable to assume that there are plenty of viruses which cause no noticeable symptoms at all in most people and therefore haven't been isolated.
posted by hattifattener at 12:35 AM on December 15, 2007

There are a shitload more viruses out there than we have actually characterised. The genomic evidence is one part, but that just means once upon a time a population of our ancestors were infected with a retrovirus that managed to put its genes into germline cells. We have no idea what viruses they were, what sort of diseases (if any) they caused or whether they're still around anymore(although there's intense interest in them right now given the possible stem-cell therapic applications, but that's another story...).

There are plenty of viruses that are seemingly harmless, but manifest into diseases when an individual becomes severely immunocompromised (when somebody has HIV, for example). Human herpesvirus 8 (which causes Kaposi's sarcoma) and John Cunningham virus (which causes progressive multifocal leukoencephalopathy) come to mind. Presumably there are more that we don't know about.

A number of autoimmune diseases are believed to arise from yet-to-be-discovered viruses. Epstein-Barr virus has proven to be a good candidate for a number of autoimmune diseases, but there are undoubtedly more.

And then there's the belief that a large number of cancers are caused by yet-to-be-discovered viruses. There are many tumors associated with known viruses (Human herpesvirus 8 as mentioned above, human papillomavirus, which can cause cervical cancer, and many others). Again, given that there are many out there that we don't know about yet, there will surely be more discoveries and disease associations described in the (hopefully not-too-distant) future.

When you get a previously unseen disease that is clearly the result of a virus, it is fairly easy to identify (the most recent example was SARS, which was promptly identified as a previously uncharacterised corona virus). So almost by definition the viruses that we don't know about are the ones that are largely asymptomatic, or have inconsistent or delayed effects (such as cancer or autoimmunity).

So the answer, most simply, is yes.
posted by kisch mokusch at 12:53 AM on December 15, 2007

Asymptomatic diseases are quite common, especially if you're willing to accept that they'll be asymptomatic in some people but have symptoms in others.

Some venereal diseases are asymptomatic in some people -- but still infectious. For instance, chlamydia is often asymptomatic.
posted by Steven C. Den Beste at 1:38 AM on December 15, 2007

(Chlamydia is bacterial, of course, but a lot of viral diseases are also asymptomatic.)
posted by Steven C. Den Beste at 1:40 AM on December 15, 2007

SIV springs to mind - the equivalent of HIV for simians. It often doesn't result in SAIDS.
posted by edd at 4:35 AM on December 15, 2007

You might also be interested in Prions.
posted by bystander at 6:21 AM on December 15, 2007

You mean a known virus that doesn't seem to cause any ill effects, right? The only ones that spring to my mind are Hepatitis G (apparently harmless) and HHV-7 (jury's still out on that one). I'm betting those two were only discovered because of their relations to more dangerous viruses. Scientists probably don't spend a lot of time looking for viruses that don't do anything.
posted by faster than a speeding bulette at 6:39 AM on December 15, 2007

"Not causing any symptoms" is somewhat of a subjective view; if (almost) every human being has it, and it causes the expression of some characteristic we didn't have before it became part of our DNA, then not having it would be, basically, a genetic disorder. This may be part of the mechanism of speciation. Sapience itself may be in this category.
posted by aeschenkarnos at 7:29 AM on December 15, 2007 [1 favorite]

Hmm Well I have been in 3 comas and Doctors have not been able to find whats wrong with me. I have had every test known to man done to me and they all come back negative.

HEre at stonybrook university i saw an article in the paper that they might have found a new virus that hides and only comes out every now and then. Of course I called them up and they had no clue about it.

All three i was normal went to sleep and woke up on life support and 15 doctors around me.

So I think there probably is atleast one virus going around that doctors dont know about and the few doctors that do know wont say anything about it.
posted by majortom1981 at 7:58 AM on December 15, 2007 [2 favorites]

Vaccines might qualify. Although they are beneficial, under ideal conditions you don't notice them at all; you simply don't succumb to the harmful variant.
posted by weapons-grade pandemonium at 10:14 AM on December 15, 2007

Experiments in which animals have been raised in completely 'germ-free' conditions have shown that bacteria (at least) are essential for normal intestinal development.

I don't know of any parallel situations with viruses, but I think it's conceivable they could exist.
posted by jamjam at 12:03 PM on December 15, 2007

I know of one virus, the JC Virus, which roughly 80 percent of the human population is infected with. It causes no symptoms unless a person has a compromised immune system. Then the virus activates and can cause death.
posted by FortyT-wo at 7:00 PM on December 15, 2007

er-- I should say the virus *can* activate. The resulting disease (PML) is quite rare even among those with compromised immune systems.
posted by FortyT-wo at 7:05 PM on December 15, 2007

Long answer: a parasitic life form evolutionairy speaking "wants to" coesist harmlessly with it's host. The long term process of evolution will naturally tend to sending the parasite and/or host extiinct, or to them coesisting peacefully.

I know this is very logical - and there's parasitic evidence of it - but that doesn't mean the principle holds true for viruses does it? I mean - it's an abstraction to say these two very different things - parasites and bacteria - are somehow functionally similar, no? I'm awful at logic games, but it seems like making assumptions about one based on the other would then be suspect?

..I'm trying to decide if it's reasonable to wonder if there isn't, for example, some 'benign' virus that's been peacefully coexisting within the human population while proliferating it's unique genetic modification. Kind of like that cat-feces mind-control parasite - if virus behavior is truly analogous to parasite behavior.
posted by unmake at 12:59 AM on December 16, 2007

Viruses aren't like parasites in that they need to use host cells to proliferate. They are not alive, so they must cause some damage when they proliferate. Which means that no virus can ever, truly, be absolutely benign. Even the JC virus mentioned above causes damage to the host. It's just not so severe as to manifest any major disease (and even then may be involved in causing cancer). Many of the Herpes virus family members are like this. You've probably had chickenpox, which means there's VZV present in your nervous system right now. And there's a good chance it will cause no more problems for you for the rest of your life, but it does cause damage during the primary infection. These viruses are really no different from all of the non-fatal viruses except their ability to persist.

But your question initially was whether there were any "invisible" viruses. i.e. something that was so mild that we (scientists) can't see or don't know about (yet). Which is a slightly different to the question of whether there're any "benign" viruses out there.

The important part of singingfish's comment was "evolutionary-speaking". He/she was coming from a population-level perspective, not an individual one. From an evolutionary perspective, any virus that is still around today has reached a kind of equilibrium with it's host population. But from the perspective of the actual people who are infected, it's a different matter altogether. There's no evidence of any peaceful coexistence between viruses and their host like, say, the relationship we have with commensal bacteria, who get to live in our nice warm stomachs and in return breaks down some of our food for us). Because a virus never gives anything back. Even the retrotransposon data doesn't support this (not at the individual level, anyway).

That being said, from an evolutionary perspective, there's growing evidence that we need viral (and other) infections to properly train our immune system, but I guess that's not really what your were asking about.
posted by kisch mokusch at 2:15 PM on December 16, 2007

It occurs to me there's another weird case. Certain parasitic wasps secret viruses that inhibit the immune system of the host it lays eggs in. The virus doesn't replicate in the host, but is part of the wasp genome.

It's not only not a disease-causing virus in the animal it 'reproduces' in (if a secreted virus can be said to reproduce?), but it's actively beneficial to that animal. That said, it is obviously dangerous for the host to the wasp larvae.
posted by edd at 8:08 AM on December 17, 2007 [1 favorite]

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