Viruses A Natural History

Viruses are possibly even more maligned than bacteria, spoken of exclusively in terms of disease. Here, virologist Marilyn J. Roossinck ranges far beyond human pathogens to convince you how narrow that picture is. She instead reveals them as enigmatic entities that are intimately entwined with the entirety of Earth's biosphere, exploiting and enabling it in equal measure. Backed by numerous infographics, the book alternates between chapters on basic principles of virology and brief portraits of noteworthy viruses. The result is an entry-level introduction to virology that fascinated me more than I expected.

This book is the fifth volume in a recently launched series called The Lives of the Natural World that has so far covered moths, fungi, beetles, and seaweeds, with more titles in the pipeline. Princeton University Press is not known for glossy popular science books full of colourful pictures and I was reminded of books by Ivy Press. Thus, with furrowing forehead and narrowing eyes, I picked this book up somewhat suspiciously: was this science-lite fluff?

Nope.

You know what? I take it all back, my initial impressions were completely unfounded. Roossinck previously cut her teeth at science communication with her 2016 book Virus: An Illustrated Guide to 101 Incredible Microbes. With the current book, she provides a nine-chapter crash course in virology that drove home how—despite having studied biology—I knew far less about viruses than I realised.

Take for instance the sheer diversity of viruses. Viruses with double-stranded DNA, single-stranded DNA, single-stranded RNA, and—wait, is that even possible?—double-stranded RNA. Viruses that infect vertebrates, plants, fungi, invertebrates, bacteria, and even archaea. Tiny viruses called viroids that do not code for proteins but whose RNA is directly active. And, hell, there are even viruses that infect viruses, so-called satellite viruses.

And what to make of the intricacies of replication? This is easily the most technical chapter in the book where having a background in biology does not hurt. It is also the most relevant as replication is what viruses are all about. Double-stranded DNA viruses rely on host enzymes to multiply. But in eukaryotic cells such as those of animals, these enzymes are only expressed during the replication phase, meaning hosts determine the tempo. Viruses can circumvent this by influencing the host's cell cycle. Double-stranded RNA, meanwhile, is a foreign substance that triggers the host's immune response, so these viruses get the host to make them a protective structure called a viroplasm where replication takes place out of sight. And some viruses are so efficient with their genetic material that their RNA codes for so-called polyproteins: large proteins that can be cut up in different ways to produce multiple functional proteins.

I was particularly fascinated by the role of viruses in evolution. I have written before about retroviruses, viruses that integrate themselves into the genetic material of their host. Roossinck points out that, since viruses have no genes in common nor leave any fossils, studying their evolutionary history is challenging. But retroviruses can mutate and become inactive, and the human genome is littered with old retroviruses. Studying them informs the discipline of palaeovirology. And how did viruses evolve? Three major hypotheses are that they evolved 1) before cellular life, 2) from a cellular organism that lost much of its genome and became parasitic, and 3) from escaped pieces of DNA or RNA. There is no compelling evidence to pick only one of these as the correct answer and Roossinck suggests that viruses possibly evolved multiple times independently. Roossinck also repeatedly writes that viruses are not "out" to make us sick. In the arms race between host and virus, all that matters for the virus is that it manages to replicate. "If that makes the host sick, too bad for the host, but disease is not a selective force in virus evolution" (p. 181). Another area where our language can trip us up is when talking about the "receptors" in the host's membrane that viruses bind to. Calling them receptors is somewhat deceptive as "the host receptor did not evolve to let the virus in and always has an important function for the host" (p. 182). Targeting such membrane-bound molecules with antiviral drugs might work but at the cost of making you sick.

Viruses furthermore do much vital work to keep our world ticking over. Bacteriophages, in particular, can be considered the good guys. At an ecosystem level, they kill an estimated 20-40% of all marine bacteria on the planet every few days, the net effect being that large amounts of organic matter remain dissolved in the upper layers of the ocean, rather than sinking to the bottom as dead bacteria and being locked away in sedimentary layers. You also find them in the mucus of our gut where they attack pathogenic bacteria and protect us from illness. With the invention of penicillin, the use of bacteriophages to combat disease, so-called phage therapy, fell into neglect. However, with the rise of antibiotic-resistant bacteria, it might just make a comeback. And for mammals to successfully form a placenta during pregnancy, they need a protein called syncytin, something for which we can thank a group of retroviruses. Lastly, she discusses numerous biotechnological tools that have been inspired by, or directly stolen from, viruses, CRISPR being a well-known recent example.

In their drive to reproduce, and given the fundamental molecular level at which they operate, viruses have inserted themselves everywhere in our biosphere. Some of the intricacies of their interactions boggle the mind. The chapters in this book alternate with short sections containing several double-page spreads that briefly introduce noteworthy viruses. For example, Roossinck's group has worked on grasses in Yellowstone National Park that can tolerate growing in hot soils thanks to colonization by a fungus infected with Curvularia thermotolerance virus (the fungus alone is not enough). Yeast cells that are infected with Saccharomyces cerevisiae virus L-A produce a toxin that kills other yeast cells. Why do the infected yeast cells not succumb to this toxin? "Simple": a satellite RNA associated with the virus deactivates the toxin.

In short, though plenty of viruses are deadly pathogens, they are also a supremely fascinating phenomenon. Viruses: A Natural History includes a large number of useful infographics that clarify many of the concepts discussed in the book. If a proper virology textbook initially seems too intimidating, this is a great entry-level stepping stone to tempt you over the threshold.