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As far as I know a virus can infect any cellular organism from a bacteria onwards, including protists, algae, plants, and so forth.

But can a virus infect a virus?

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No, it cannot. Viruses infect cells, in order to use the cellular machinery to replicate themselves - viruses themselves do not possess such a machinery, so speaking of one virus infecting another is meaningless.

This does not mean that viruses do not interact: when two viruses co-infect the same cell, the progeny virions may contain parts of the genomes of both viruses. This is how genetic recombination occurs in HIV.

An even more famous example is gene reshuffling of influenza viruses, where an animal virus exchanges its genes with a human virus, producing a new human strain. In the popular culture some of these a famously named after animals whose virus contributed or animals where the reshuffling has occured (bird's flu, swine flu, etc.)

Update
It is worth mentioning the satellite viruses, which are viruses that lack the proteins necessary to catalize the replication and possibly even the genes coding for their capsid proteins (in which case they are more properly called satellite nucleic acids rather than satellite viruses). These viruses are dependent on a helper virus to carry the missing genes. This is not characterized as parasitism, but it is rather close.

Update 2
A special type of satellite viruses, called virophages, actually do exhibit characteristics of parasitism in respect to the helper virus:

Unlike satellite viruses, virophages have a parasitic 
effect on their co-infecting virus. Virophages have been 
observed to render a giant virus inactive and thereby 
improve the condition of the host organism.
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