This question has come up in my life innumerable times in the last 40 years that I have been interested in viruses – and I find that debate on it often becomes theological in its intensity, with proponents muttering things like “They’re just intracellular parasitic molecules!”, or “Of course they’re alive – they evolve, don’t they?”
And as happens when I hear theological arguments, my eyes glaze over, and I think of other things.
Because it’s really quite simple: as I have been patiently explaining to students for over thirty years now, viruses are simply acellular organisms – which find their full being inside host cells, where some measure of essential support services are offered in order to keep the virus life cycle turning. What everybody sees as “viruses” are in fact virions, the particles that viruses cause to be made in order to transport their genomes between cells, and to preserve them while doing so. Thus, in a very real sense a virus IS the cell it infects – because it effectively takes the latter over, and uses it to make portable versions of the genome that can infect other cells.
Back in 1995 or so, I wrote the following for my first teaching pages:
The concept of a virus as an organism challenges the way we define life:
- viruses do not respire,
- nor do they display irritability;
- they do not move
- and nor do they grow,
- however, they do most certainly reproduce, and may adapt to new hosts.
By older, more zoologically and botanically biased criteria, then, viruses are not living. However, this sort of argument results from a “top down” sort of definition, which has been modified over years to take account of smaller and smaller things (with fewer and fewer legs, or leaves), until it has met the ultimate “molechisms” or “organules” – that is to say, viruses – and has proved inadequate.
If one defines life from the bottom up – that is, from the simplest forms capable of displaying the most essential attributes of a living thing – one very quickly realises that the only real criterion for life is:
The ability to replicate
In fact, I have just uncovered something I started writing in 2005 on the same subject:
Fruits of the Bushes of Almost-Life: A Natural History of Viruses
It is an amazing thing that when people speak or write of “life”, they generally miss out one of the most interesting and diverse facets of it: viruses…!
Viruses are like the black sheep of the family of life: they are everywhere, they infect everything, they are the most diverse organisms on our planet, and yet most biologists either do not mention them, or simply dismiss them as “parasites that you can’t even see with a microscope”.
Consider this: of the seven different kinds of genetic material shared by all organisms, all cellular life has only one – and viruses have all seven.
And this: the most abundant and genetically diverse organisms in the oceans, and therefore probably on the planet, are viruses.
Or this: viruses may be the only bridge left to understanding how our DNA-dominated cellular world came to be, from primitive RNA-genomed ancestors.
Let’s face it, viruses generally get a bad press: if it’s not “Will avian flu kill us all?”, it’s “Marburg virus outbreak threatens Luanda”, or “Is SARS coming back?” leaping out at us from our local newspapers – and that’s all just this year. We even have speculation that global warming could unleash long-frozen plagues on us, as viruses thaw out of the Greenland icecap – and while all of this represents media hype, there are grains of truth in all of it, Yes, the H5N1 influenza virus epidemic in poultry is a matter for very serious concern; yes, there was a chance that Marburg virus – an equally nasty relative of the dreaded Ebola – could have devastated Angola’s capital as recently as a few months ago; yes, epidemiologists are worried that SARS coronavirus may again leap out of its animal or even possibly human reservoirs and into the world and kill thousands; yes, long-frozen viruses may yet represent an unexpected and unwelcome disease threat to humanity, its crops and its livestock.
However, all of these concerns highlight only one facet of the complex phenomenon that is viruses: that is, the “viruses as ogre” side of these organisms. Not that this is not amply justified: the single biggest killer of humans this year will probably be either diarrhoea-causing viruses or HIV; the legendary Black Plague that repeatedly decimated medieval Europe may well have been a haemorrhagic fever virus rather than a bacterial disease, and the “Spanish Flu” pandemic of 1918-1922 is now known to have killed more than 60 million people. But viruses also possibly gave us the ability to develop a placenta and develop away from marsupials to become mammals – and viruses probably also regulate the lifetimes of algal blooms, cholera epidemics, aphids, moths that attack conifers, and possibly every living thing in the seas.
In short, viruses are intimately intertwined into every ecological web on this planet, whether we know it or not – and we find out more and more how much this is true the more we look.
What are Viruses?
The very nature of viruses severely taxes all conventional notions of what is an organism, or even of what is life. While the debate on whether viruses are living or are indeed organisms gets almost theological in its intensity in certain biological circles, there is a very simple way around the problems – and that is to regard them as a particle/organism duality, much as physicists have learned to do with the dual wave/particle nature of light.
Quite simply, viruses are obligate intracellular parasites which use the resources of living cells to multiply their genetic material, and to make specialised particles which serve to protect and transport the genetic material, or genome, to other susceptible cells. Their dual nature is defined by the two ends of their life cycle: the virus as organism is inextricably mixed into infected cells, integrated into pathways of nucleic acid and protein synthesis; the virus as particle can be purified away from all cellular components and kept in a bottle like a chemical, totally inert, until you decide to reintroduce it to its host cells.
“The idea that virus and virion are distinct was first proposed by Bandea in 1983. He suggested that a virus is an organism without a cohesive morphological structure, with subsystems that are not in structural continuity…Viruses are presented as organisms which pass in their ontogenetic cycle through two distinctive phenotypic phases: (1) the vegetative phase and (2) the phase of viral particle or nucleic acid. In the vegetative phase, considered herein to be the ontogenetically mature phase of viruses, their component molecules are dispersed within the host cell. In this phase the virus shows the major physiological properties of other organisms: metabolism, growth, and reproduction”
OK, I think I said pretty much the same thing in teaching in 1981, but less elegantly, and I have no proof other than ooooold overhead projection slides B-) He goes on, though, to mention in the context of mimiviruses, that
“Claverie suggested that the viral factory corresponds to the organism, whereas the virion is used to spread from cell to cell.”
This crystallises things nicely: viruses are acellular parasites which take over a cell, and make specialised particles (virions) to spread their genomes. Qualitatively, this is exactly what seeds and spores of plants and fungi do: they make specialised vehicles that preserve their genomes, and which can respond to changes in their environment to initiate a new organism.
However, it’s in his last point that Racaniello gets sufficiently theological to silence some of the doubters. He writes:
“Raoult and Forterre have therefore proposed that the living world should be divided into two major groups of organisms, those that encode ribosomes (archaea, bacteria and eukarya), and capsid-encoding organisms (the viruses).”
I like that. I like it a lot. It makes a lot of sense. And in the light of my last two posts in ViroBlogy – on “The Bushes of Life“, and Deep Evolution of Viruses – I can see that the time has come to spread The True Gospel of Virology.