Thread by @eugyppius1

This is a tobacco leaf afflicted with so-called tobacco mosaic disease.
At the end of the 19th century, people noticed that you could crush up these diseased leaves, bring them into contact with healthy tobacco plants, and the healthy leaves would soon exhibit the same disease.

This was noted at the end of the 19th c., when the study of bacteria was very cutting edge. Bacteria were at that point the only causative agent for transmissible pathogens that had been identified. Microscope tech was good enough then, that bacteria could be observed.

A number of different scientists, including Pasteur, began to suspect that bacteria could not be the only causative agent here. Nobody could find any rabies 'bacteria,' for example, despite extensive searching for bacteria in samples from rabid animals.

Also, nobody could find bacteria in leaves suffering from tobacco mosaic disease ... and yet, as I said, the disease appeared to be transmissible from plant to plant.
Enter the Chamberland filter, invented in 1884. It was capable of filtering out bacteria-sized particles.

In 1892, a Russian botanist named Dmitri Ivanovsky forced infected tobacco plant through a Chamberland filter, and used the filtered (& theoretically bacteria-free) sap to infect a healthy plant with tobacco mosaic disease.

This is generally considered the first moment that the action of a non-bacterial infectious agent was conclusively demonstrated, though as always, these processes of discovery are a little more complicated than that.

Ivanovski also noticed peculiar pathological features in the cells of affected tobacco plants("intracellular inclusions"), which he described for the first time in this article.

Despite this clear evidence of some kind of sub-bacterial cell-infecting pathogen, he continued to insist that tobacoo mosaic disease must be caused by an unusually small submicroscopic bacteria.

A few years passed. A Dutch Botanist named Martinus Beijerinck became interested in the whole issue. He started out by replicating Ivanovski's experiments with the Chamberland filter. He chracterised the filtered fluid from infected plants as a "living contagious fluid"

His perceptive and intelligent article is online here, and a real joy to read. He noted, for example, that the filtered sap could infect an effectively limitless number of healthy plants and that it must reproduce in its hosts (i.e., is not a toxin).
dwc.knaw.nl/DL/publication…

He put a drop of filtered, infected sap in an agar plate (a petri dish with a growth medium). If he waited a few days, he found he could infect healthy plants in a somewhat wider radius, than where he had put the drop – i.e., the pathology was caused by water-soluble particles...

...that could diffuse in a liquid medium. Yet, it didn't seem to grow in the sugars of the petri dish, like bacteria would. he could tell, because when he infected healthy plants from the agar plate weeks and weeks later, they didn't suffer from more virulent disease.

He could even dry out the medium in which he had placed the infectious sap, and it would still infect healthy plants even months later, b/c tobacco mosaic virus is non-enveloped and fairly durable. This too had no effect on disease severity.

All of this was really bizarre, for scientists used to studying bacteria. If you culture the bacteria for a while in a petri dish, the concentration of bacteria should increase, causing more severe infection in a host. If you dry out the medium ...

this should likewise kill or at least attenuate bacteria infectiousness. Beijerinck had discovered that the tobacco pathogen a) didn't replicate on its own, was b) to some degree oddly durable, not like a microorganism, & c) could cause the same severity of disease ...

regardless of concentration. The latter, of course, is down to the fact that viruses hijack healthy cells to reproduce themselves. Infected cells produce so many virions, it's hard to detect differences in infection severity between infections caused by few virus particles...

...and infections caused by many. Beijerinck christened this pathogen a "virus," and the term stuck. Everything else has been details.
Thus we see, that the existence of some virus-like pathogen was anticipated by multiple researchers before the actual thing was discovered ...

because bacteria were insufficient to explain all observed transmissible pathogens; and we also see that in vivo transmissibility was absolutely central to the discovery of submicroscopic viruses. Only by causing mosaic disease in healthy plants ...

could people like Beijerinck demonstrate that there was virus in a sample at all.
Early research on viruses is highly interesting and completely replicable by total amateurs today. You can describe most of the basic properties of viruses by conducting these same experiments.