Are viruses alive? (and other old chestnuts)

Deep into Carl Zimmer's book Life's Edge: The Search for What It Means To Be Alive, it was only a matter of time before the issue of viruses had to be broached. 

Ask a scientist whether viruses are alive and you are likely to get an embarrassed cough or a roll of the eyes, and a "We don't really know", or "That depends", or more specifically, "That depends on your definition of 'life'". A few may be bold enough to say "Yes" or "No", and may even be able to justify their answer. But the very fact that some say "Yes" and some say "No", however confidently, reveals that the jury is most definitely still out.

Even a definition of life is tough to narrow down. The so-called NASA definition - "Life is a self-contained chemical system capable of undergoing Darwinian evolution" - is a very careful formulation that hopes to avoid many of the pitfalls previous definitions have fallen into. So, are viruses "alive" under this definition? Well, "we don't really know" or "that depends"...

There are literally trillions of viruses in a human body, most of them not at all dangerous (and many even positively beneficial) to human life and health. Most of them infect the trillions of bacteria, fungi and other single-celled organisms that make up our microbiome, and indeed help to keep our microbiome in balance, paradoxically keeping us healthy. 

Also, there are literally trillions of different species of viruses in the world, many of which have adapted to very specific niches. There are more viruses in a litre of seawater or a spoonful of dirt than there are humans on earth. There are more viruses on earth than every form of cell-based life combined, perhaps by a factor of ten. Viruses are tiny, much smaller than even the smallest bacteria, but we now have powerful electron-microscopes that can see them. So why is it so hard to decide whether or not they are alive?

Bacteria are single-celled organisms that can generate energy, make their own food, move, and reproduce themselves (usually by binary fission, in which DNA is duplicated and carried down, much as it is in our own cells). Bacteria, it is widely agreed, are alive. Although viruses do exhibit some of the hallmarks of "life", and they certainly do evolve (as we have seen with COVID-19), they can only grow and reproduce inside the host cells they infect. They are essentially parasitic and, on their own, they just remain dormant. They do not eat or grow as such; they do not use enzymes to metabolize like our cells do; yes, they replicate, voraciously, but they do not really reproduce in the same way that we think of reproduction (a virus is just a "reorganized package of its host's own atoms"); some do not even contain DNA, having evolved to transfer their genes directly through RNA.

Wait, isn't DNA an easy way to decide whether something is alive or not? All cellular life we know of - animals, plants, bacteria, even archaea - uses the more stable double-stranded DNA genomes to transfer their genetic material. But, while some viruses also have DNA, many viruses use single-stranded RNA and still do a perfectly good job of passing on their genes, and it is thought that very early life on earth probably started out with RNA. Are RNA-based viruses any less alive than DNA-based organisms, then? Are some viruses alive and some not? You can easily run around in circles with these questions. 

And, of course, the very definition of life that scientists are working around is subject to debate, and there are many other alternative formulations, some more constrained, some substantially freer, than the NASA one (one study identified 123 different scientific definitions of life).

Anyway, as a result of all these considerations and more, many scientists consider that viruses only lead a kind of "borrowed life" or a "half life". Either way, you can see why this is such a fraught question in science. Of course, you might also say, "Does it really matter?" Just don't say it to a biologist or biochemist.