Carl Zimmer's primer on viruses entertains, but reveals little about their basic traits, says Robin Weiss.
A Planet of Viruses
By Carl Zimmer
Viruses propagate in every kind of living organism and, despite being so tiny, amount to around 5% of the world's biomass. Viruses that infect algal blooms, for example, induce the calcification of their hosts and are thus responsible for the white cliffs of Dover in Britain, and other carbon sinks. Acclaimed science writer Carl Zimmer celebrates the versatility of these agents in a dozen entertaining essays in his latest book, A Planet of Viruses, which accompanies the World of Viruses educational website (http://www.worldofviruses.unl.edu).
Each essay deals with a different virus, ranging from those in bacteria and plants to scourges such as smallpox, severe acute respiratory syndrome (SARS), influenza and West Nile virus. There is food for thought here for all, even a seasoned virologist like me. Zimmer's writing grabs one's interest, but is marred by a lack of attention to detail. It would be a more attractive little volume if it were half as long again, because many of the essays end just when they become interesting.
Zimmer begins with the observation in 1898 by the Dutch microbiologist Martinus Beijerinck that the tobacco mosaic disease of plants was caused by “a contagious living fluid”. Dmitry Josifovich Ivanovsky is ignored in the account — even though he had also isolated and propagated tobacco mosaic virus from filtered plant sap six years earlier — because he thought it must be a bacterial disease. The property of transmission through a filter too fine for bacteria to pass through became the defining feature of a virus, thanks to Beijerinck but also to Friedrich Loeffler and Paul Frosch, who reported the filterable nature of foot-and-mouth disease in the same year.
Zimmer omits the marvellous subsequent discoveries using tobacco mosaic virus: its crystallization from the 'living fluid' by Wendell Stanley in 1935; its composition as protein wrapped around RNA by Norman Pirie and Frederick Bawden in 1936; the first visualization of virus particles through the electron microscope by Gustav Kausche, Edgar Pfannkuch and Helmut Ruska in 1939; and the first demonstration, by Heinz Fraenkel-Conrat in 1955, that RNA alone can reconstitute infection. One of the most fascinating tales of twentieth-century science is how viruses opened up molecular biology, but you won't find it here.
This year marks the centenary of Peyton Rous's demonstration that a cancer in chickens can be transmitted by a filterable agent. Zimmer's essay 'Rabbits with Horns' recalls Rous's pioneering work on tumour viruses, before introducing Richard Shope's discovery of the rabbit papilloma viruses, to which the essay title refers. Harald zur Hausen and colleagues discovered human cervical papilloma viruses in 1983; the vaccines that target them to protect women against cervical cancer were licensed in 2006. Thus, 100 years of tumour virology paid off. Rous may have had to wait longer than any other Nobel laureate to win his prize in 1966, whereas zur Hausen's 'incubation period' was a mere 25 years.
Owing to Zimmer's puzzling reluctance to delve into molecular virology, we have to wait until the end of the last and best essay, on the giant mimiviruses — discovered only in 1992 — to learn that some viruses have RNA genomes instead of DNA. No other replicating systems carry their genes in the form of RNA, as do polio, measles, influenza and most plant viruses. Some viruses have double-stranded RNA, whereas others are single-stranded; some viruses carry a single RNA or DNA molecule, and others have segmented genomes like the different chromosomes of higher organisms. Neither of the two chapters on retroviruses mentions reverse transcription — by which the RNA genome is turned into DNA before inserting itself into host DNA — even though the most potent anti-HIV drugs are designed to block this process.
Perhaps Zimmer thinks such facts are too difficult for his readership, but I view avoiding them as dumbing down. Which viruses evolved from bacteria, and which are more likely to have emerged as sets of genes that escaped from their hosts? Are some viruses relics from an RNA-based world, or are they relatively modern parasites derived from other living systems? Zimmer eventually raises the last question, but to my mind, the fascination of viruses is their enormous molecular and evolutionary diversity as much as their pervasiveness in the environment.
Concern for accuracy seems to have suffered as Zimmer becomes an ever more prolific writer. Some virologists' names are wrong, for instance, as are some other simple facts.
In a foreword to Peter Medawar's 1996 collection of essays, Stephen Jay Gould called this literary form “a weapon of wit and instruction”. It would be difficult for any science writer to match Medawar or Gould; nevertheless, Zimmer's contributions fit this definition well.