VIRUSES ARE LIKE ZOMBIES: they are not quite dead and, to keep going, they have to feed on living flesh. But they are not quite alive either. When a bacteriologist wants to needle a virologist, she dismisses viruses as “infectious chemicals”, not properly living things. Properly living things are made of cells, membrane-bound bags of vibrant chemical reactions. At best, viruses can be characterised as “minimal biological entities”.
In classical Latin, the word virus was applied to any poisonous secretions or venoms, or metaphorically to anything with a virulent or malignant quality. Cicero uses it this way in his De Amicitia, with his turn of phrase evomat virus acerbitatis suae meaning “spits out the venom of their own bitterness”. In English, the word had started to take on a more modern meaning as "material that causes infectious disease" by the eighteenth century. Edward Jenner in his groundbreaking text from 1798 AN INQUIRY INTO THE CAUSES AND EFFECTS OF THE VARIOLÆ VACCINÆ uses the term “virus” more than twenty times to describe the infectious material that causes cowpox.
Louis Pasteur, one of the founders of medical microbiology, is buried in a neo-Byzantine chapel within the Institut Pasteur at the heart of Paris. The walls of the crypt are lined with exquisite marble, above which sit mosaics providing vignettes from Pasteur’s highly productive life. The penultimate vignette carries the inscription “1880 virus vaccins” and refers to Pasteur’s work on the production of artificial vaccines against chicken cholera and anthrax. The final inscription refers to Pasteur’s work on vaccination against rabies.
Pasteur was puzzled by rabies. He could grow and/or use a microscope to visualise the causative agents of most of the infectious diseases on which he worked—these agents we now call bacteria. But the cause of rabies eluded him—and for good reason, as we now know it was caused by a virus rather than a bacterium. However, one of his staff, Charles Chamberland, laid the groundwork for the discovery of what we now call viruses, by inventing the Chamberland-Pasteur filter. This filter, made from unglazed porcelain, had such a small pore size (less than a millionth of a metre), that it could be used to remove bacteria from a liquid suspension.
In the early days of the new Germ Theory of Infection, it was thought that all infectious agents could be retained by filters and grown on a nutrient medium. However, in the last decade of the nineteenth century, it became clear that some infectious agents did not comply with these expectations. In 1892, Dmitri Ivanowski, working in St Petersburg, used a filter candle on an extract of plants with tobacco mosaic disease and showed that, even after filtration, the extract remained infectious. But he didn’t quite get clear in his head what had happened—he thought perhaps what he was seeing was a toxin produced by a bacterium.
Six years later, the Dutch scientist Martinus Beijerinck described his own similar experiments with infectious bacteria-free extracts of tobacco mosaic disease, but this time made the conceptual leap to the idea of a new kind of infectious agent, which he termed “filterable virus” that would grow only in the presence of living cells. The adjective was soon dropped and modern usage of the term “virus” became widespread.
In 1906, the Italian pathologist, Adelchi Negri showed that Jenner’s vaccine virus was also a “filterable virus” and soon afterwards, the causative agent of smallpox was also shown to be viral in nature.
But what aside from size makes viruses different from bacteria?...