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A tobacco plant (1935)

In the late 1880s, a young botany student from St Petersburg University was sent off to the Ukraine and Russian Bessarbaria – now known as Moldova – to investigate the causes of “wildfire”, a disease affecting the tobacco plantations of the Tsarist empire. Dmitri Ivanovsky went on to study a similar plant disease affecting the tobacco crops of the Crimea and in both cases he found they were caused by a mysterious infectious agent that did not follow the normal rules of bacterial disease.

These infectious agents, whatever they were, easily permeated the porcelain Chamberland filters used to capture bacteria. Ivanovsky discovered that the filtered sap of infected plants was still capable to passing on the “mosaic” leaf disease to healthy plants and he concluded that the causal agent must be an extremely small parasitic microorganism that was quite invisible under the strongest magnification available at the time.

A few years later, a Dutch microbiologist called Martinus Beijernick, working independently of Ivanovsky at the Technical School Delft, identified an important property of this new kind of infectious agent – it was able to migrate in an agar gel rather than becoming fixed as it would be if it were a bacterium. Beijernick said it must therefore be an infectious soluble agent, or “contagium vivum fluidum” rather than “contagium fixum”. In 1898, he was the first to call this new class of infectious agents “viruses”.

However, the real breakthrough in studying the virus came in 1935 when Wendell Meredith Stanley of the Rockefeller Institute for Medical Research in Princeton, New Jersey, became the first scientist to purify and isolate the tobacco mosaic virus (TMV) from the leaves of the tobacco plant. He managed to prepare crystals of TMV from a concentrated filtrate of plant sap which allowed the virus to be seen for the first time under a microscope at about 400 magnifications. Wendell went on to show that TMV was composed of protein and ribonucleic acid, or RNA – a genetic instruction code wrapped in a protein coat. Until this point, “it was not known whether viruses were inorganic, carbohydrate, hydrocarbon, lipid, protein or organismal in nature”, Wendell explained in his Nobel acceptance lecture of 1946.

“The fact that, with respect to size, the viruses overlapped with the organisms of the biologist at one extreme and with the molecules of the chemist at the other extreme only served to heighten the mystery regarding the nature of viruses. Then too, it became obvious that a sharp line dividing living from non-living things could not be drawn and this fact served to add fuel for discussion of the age-old question of ‘What is life?’,” Wendell said.

We now know that viruses, based on either DNA or RNA, are capable of hijacking the cellular machinery of protein replication within a host organism. Much of the human immune defences are geared towards combatting these smallest and deadliest of parasitic foes.