1) Even though big populations make huge pandemics possible, they also offer a vast diversity in our genetic code. Basically this means that there is a chance of some virus wiping out a big portion of the human race, but not all of us, since some people are almost certain to carry a protective mutation against that given virus. We can already see this with the HIV: in some regions protective mutations to the prevalent HIV strain have emerged, though in very few individuals so far. But if the pressure is big enough, these people will be likely to increase in numbers.

If human populations were small, huge wiping-out pandemics could not happen. Before the start of agriculture, global or even continent-wide epidemics did not really happen.

2) As far as I know, there is no known virus that could infect all living things. To the contrary, vast majority of viruses are very host-specific and target cell-specific, and cannot normally cause disease in any other organisms. Some notable exceptions include viruses that cause the so-called zoonoses, which are animal diseases that can in some cases contract to humans (this can happen the other way round as well). However, viruses causing zoonoses are "poorly designed" to live in other organisms than their normal hosts and thus often cause too fulminant infections, sometimes even killing the new host. But considering the vast amount of virus species/strains there are, only a small fraction of them can cause cross-species infections.

Bacteria that cause zoonoses are more common, because many bacteria are typically far less species-specific than viruses are. Being exclusively parasites, viruses target very specific structures (often receptors) on their target cell’s surface that make them unable to infect cells that lack the given target. Pathogenic bacteria, to the contrary, do not depend as much on such specific targets and thus can have a wider spectrum of hosts, or they can cause opportunistic infections even if they normally were free-living non-pathogens in the soil or in some animal’s gut etc.

When life was very young on this planet and organisms much more uniform in function there might have been more wide-spectrum viruses or virus-like particles.

The "gap" is probably the fact that there is no virus that would have a means of entry to all possible host cells in the world (because of the diversity in target receptors and such). Also, cells have many ways of destroying foreign DNA that manages to enter it, and this all-infecting virus would have to evade all such mechanisms as well. Finally, viruses do not really attack the DNA in cells anyway, even though some species can integrate into it.

Curiously though, it seems that we all carry viruses or viral DNA in our genomes. In theory it is possible that some piece of the DNA of most of the organisms could be originally an integrated virus. Just as we carry mitochondria (and plants chloroplasts) that were originally prokaryotes, there could be viral DNA that actually encoded something useful for their host cell and so has remained in place for as long as mitochondria – millions of years.

Yeah, that is what I meant. I do not know if this has been proved or disproved scientifically, but at least at the moment I do not see why it could not happen.

But again, viruses generally do not target DNA. They enter the cell’s cytoplasm and express their genome there, producing building blocks for new viruses just as if they were the cell’s own molecules. Only some viruses integrate into the host genome, and I believe the main function of that is to remain dormant and protected from outside threats such as viral-DNA-degrading enzymes of the host cell or (nowadays) antiviral drugs. These viruses then sometimes re-activate and start producing new viruses.

That being said, it is also a convenient way for the virus to replicate its genome if it is integrated into a dividing cell. But only in the germ line cells the viral genome can be transferred along with the host from generation to the next. I am not aware if any of the currently known viruses actively seek to infect germ line cells and integrate into their genomes in order to multiply their DNA along with the host’s. The idea is interesting, though.

Just curious, what is the theory on the origin of viruses? Remnants of prokaryotic DNA?

There is a virus required for formation of placenta in sheep. The virus helps the cells fuse to form a syncytium.

Dunlap KA, Palmarini M, Varela M, Burghardt RC, Hayashi K, Farmer JL, Spencer TE. Endogenous retroviruses regulate periimplantation placental growth and differentiation. Proc Natl Acad Sci U S A. 2006 Sep 26;103(39):14390-5. Epub 2006 Sep 15.

There is a similar virus required in humans — I can get a reference if anyone wants to follow up on that.

Very interesting reading. I have not come across endogenous retrovirus in my developmental studies. Then again, I have not studied developmental biology for so long to know what is beyond trophoblast/placental/extraembryonic material.

Wow, that’s some interesting stuff. I knew we had viral DNA in our genomes, but that they could actually benefit us was just my own guess. I am very pleased to notcie that my guess wasn’t all that bad – some viruses are beneficial for their hosts in many interesting ways 🙂

On the other front, pathogens almost always evolve quickly to produce milder forms of disease – a pandemic will tend toward less-and-less lethality. A pathogen strain that keeps a host up-and-about will spread much more efficiently than one that quickly disables the host. Pathogens wiping out whole populations would require a particular type of population: confined (domestic animals are vulnerable, especially on modern factory farms, and cities are bad places to be) or already vulnerable (distemper might wipe out African lions, who are already in serious trouble).

lol, Hiology, i digg that, Mind if i use it?