Discovery

Discovery

The existence of viruses first came to light through a puzzle involving diseased tobacco plants. In 1892, Dmitri Ivanowsky found that sap taken from tobacco plants showing mosaic disease remained infectious even after it had been passed through a fine porcelain filter known to trap ordinary bacteria - implying that whatever was causing the disease was smaller than any known bacterium. In 1898, Martinus Beijerinck confirmed and extended this finding, and proposed that the infectious agent was a new kind of entity altogether, which he described as a 'contagium vivum fluidum' - a soluble, living germ distinct from bacteria. This was, in effect, the founding of virology, the study of viruses. Decades later, in 1935, Wendell Stanley succeeded in crystallising the infectious agent responsible for tobacco mosaic disease - now known as the Tobacco Mosaic Virus (TMV) - and showed that it was made up largely of protein. Stanley's work was significant because it showed, for the first time, that a virus could be isolated and studied as a definite chemical substance, bridging the gap between the idea of a 'living germ' and the tools of chemistry.

These discoveries also gradually revealed why viruses occupy such an unusual position in biology, straddling what are normally thought of as the living and the non-living worlds. Outside a host cell, a virus particle is chemically inert - it shows none of the signs of life such as growth, movement, or independent metabolism, and can even be crystallised like an ordinary chemical compound, as Stanley demonstrated with TMV. Once it enters a suitable living host cell, however, the same virus particle can direct the host's own cellular machinery to make many new copies of itself, a capacity for multiplication that is normally considered a defining feature of life. It is precisely because a virus shows no living properties on its own, yet can multiply once inside a host, that viruses are usually described as being on the borderline between living and non-living matter, rather than being placed cleanly into either category.