I read several different articles about virology, including the Wikipedia article on viruses and none of them explain how a virus is isolated. Some talk of injecting a virus into a chicken egg, but this does not isolate the virus, it just reproduces it.

If a researcher filters the particles in a serum to get those of a particular size, it proves nothing, because every creature has many thousands of different viruses in their blood, so any sample filtered by size will potentially have hundreds of different viruses in it, all the same size range. How can they possibly be separated?

There are many ways. In many cases especially traditionally viruses of interest were either the only virus present, or overwhelmingly more abundant than other types, so simple dilution will end up with a single virus type. Differential growth can help; many viruses have very specific growth requirements and if you only offer a particular cell type (or an egg -- most viruses do not grow in eggs!) it's not unlikely that only a single virus will grow out. Those are probably the most common and simple approaches -- dilution, or attempted growth with many different cell types/conditions followed by dilution. If they don't work, it becomes a lot tricker. Complexities in Isolation and Purification of Multiple Viruses from Mixed Viral Infections: Viral Interference, Persistence and Exclusion (pointed out by @CMosychuk in comments) reviews some approaches.

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