A Planet of Viruses - Carl Zimmer [2]
Over the centuries, virus took on another meaning: it signified any contagious substance that could spread disease. It might be a fluid, like the discharge from a sore. It might be a substance that traveled mysteriously through the air. It might even impregnate a piece of paper, spreading disease with the touch of a finger. Virus only began to take on its modern meaning as the nineteenth century came to a close, thanks to an agricultural catastrophe. In the Netherlands, tobacco farms were swept by a disease that left plants stunted, their leaves a mosaic of dead and live patches of tissue. Entire farms had to be abandoned.
In 1879, Dutch farmers came to Adolph Mayer, a young agricultural chemist, to beg for help. Mayer carefully studied the scourge, which he dubbed tobacco mosaic disease. He investigated the environment in which the plants grew—the soil, the temperature, the sunlight. He could find nothing to distinguish the healthy plants from the sick ones. Perhaps, he thought, the plants were suffering from an invisible infection. Plant scientists had already demonstrated that fungi could infect potatoes and other plants, so Mayer looked for fungus on the tobacco plants. He found none. He looked for parasitic worms that might be infesting the leaves. Nothing.
Finally Mayer extracted the sap from sick plants and injected drops into healthy tobacco. The healthy plants, Mayer discovered, turned sick as well. Some microscopic pathogen must be multiplying inside the plants. Mayer took sap from sick plants and incubated it in his laboratory. Colonies of bacteria began to grow and became large enough that Mayer could see them with his naked eye. Mayer applied the bacteria to healthy plants to see if it would trigger tobacco mosaic disease. It failed. And with that failure, Mayer’s research ground to a halt.
A few years later, another Dutch scientist named Martinus Beijerinck picked up where Mayer left off. He wondered if something other than bacteria was responsible for tobacco mosaic disease, something far smaller. He ground up diseased plants and passed the fluid through a fine filter that blocked both plant cells and bacteria. When he injected the clear fluid into healthy plants, they became sick.
Beijerinck filtered the juice from the newly infected plants and found that he could infect still more tobacco. Something in the sap of the infected plants—something smaller than bacteria— could replicate itself and could spread disease. Beijerinck called it a “contagious living fluid.”
Whatever that contagious living fluid carried was different from any other kind of life biologists knew about. It was not only inconceivably small but also remarkably tough. Beijerinck could add alcohol to the filtered fluid, and it would remain infective. Heating the fluid to near boiling did it no harm. Beijerinck soaked filter paper in the infectious sap and let it dry. Three months later, he could dip the paper in water and use the solution to sicken new plants.
Beijerinck used the word virus to describe the mysterious agent in his contagious living fluid. It was the first time anyone used the word the way we do today. But in a sense, Beijerinck simply used it to define viruses by what they were not. They were not animals, plants, fungi, or bacteria. What exactly they were, Beijerinck could not say. He had reached the limits of what nineteenth-century science could reveal.
A deeper understanding of viruses would have to wait for better tools and better ideas. Electron microscopes allowed scientists to see viruses for what they are: particles of a nearly inconceivably small size. For comparison, tap out a single grain of salt from a shaker. You could line up about ten skin cells along one side of it. You