This begs bigger questions, though—like what makes viruses so adaptable, and are they really “alive?”
First, their total number is staggering. It is estimated that there are 10 viruses for every bacterium on Earth. Curtis Suttle from the University of British Columbia in Vancouver compared the number of viruses in the oceans alone to the number of stars in the Universe, which is estimated to be 1023. Viruses outnumber stars by a factor of 10 million. If you lined them all up, that line would be 10 million light years long! To put it on a more conceivable scale, it’s been estimated that each day, more than 700 million viruses, mainly of marine origin, are deposited from Earth’s atmosphere onto every square meter of our planet’s surface.
The diversity of viruses is just as impressive. Some use DNA to pass on genetic information, some use RNA, and some use both during their life cycle. The information carrier can be single-stranded, double-stranded, or double-stranded with some regions being single-stranded. Viruses are like a natural lab seemingly playing around with genetic permutations and combinations. While most viruses are so small that they can only be observed directly with an electron microscope, others, like the giant Mimiviruses, reach the size of bacteria. When I worked in my lab with viruses that kill bacteria—called bacteriophages—we did not count the actual viruses, but the number of bacteria they killed.
So, are viruses alive?
It depends where we draw the border between non-life and life, which is likely a continuum toward increasing complexity. Does life require cells? Personally, I think that’s a bit—how should I say it?–cell-centric rather than Earth-centric. In my view, viruses have to be counted as alive. We should recognize them as a fourth domain of life and not dismiss them, if only because they do in fact reproduce outside their own “bodies.” The parasitic bacteria that cause chlamydia are considered to be living. One hypothesis for the origin of viruses says that they, or at least some of them, could have evolved from bacteria that lost any genes not needed for parasitism. If so, could we say they “evolved” from living back to non-living?
Another thing I find intriguing: The more common RNA viruses—like the coronavirus behind the current pandemic—have typically smaller genome sizes than DNA viruses, apparently because of a higher error-rate when replicating. Too many errors have the effect that natural selection disfavors them. It also limits the maximum size of these viruses.