Beijerinck · The virus
Every germ is filtered out — so why is the disease still there?
That time, that place
In the 1890s tobacco was a money crop. But a disease was going round the fields: leaves went blotchy and withered. The blotches looked like mosaic, so it was called mosaic disease.
Koch's method was the fashion. A disease means a germ; find the germ, grow it alone, and you have your culprit. A young Russian scientist, Ivanovsky, set out to catch the tobacco germ that way.
Why this question
He squeezed juice from sick leaves and passed it through a fine porcelain filter. Its pores were too small for any bacterium. The filtered liquid should have been clean.
He rubbed it on healthy leaves. They fell sick.
Ivanovsky assumed the filter leaked. A few years later Beijerinck in Delft repeated the experiment. The filter did not leak.
What was found
Beijerinck saw something stranger. However much he diluted the filtered juice, it still sickened leaves, and juice from those leaves spread the disease again. It did not get weaker. It had **multiplied inside the leaf.** A poison would thin out. Only living things multiply.
Yet it could not be grown on a dish like a bacterium. It multiplied only inside a living leaf.
He called it a 'contagious living fluid', and named it with the Latin word for poison: virus. Smaller than a bacterium, multiplying only inside another's cell. Its face was not seen until the electron microscope, forty years later.
All infections are caused by bacteria, and bacteria are stopped by a filter
Juice that passed a bacteria-proof filter still spread disease, and multiplied inside the leaf
In 1935 Stanley crystallised this virus like salt and kept it in a jar. Dissolved years later, it spread disease again. Can something that forms crystals be alive? The argument has never ended