The polymerase in most RNA viruses is an RNA-dependent RNA polymerase, an enzyme that makes copies of RNA. So what happens is: a single piece of viral RNA comes in the cell. Then it gets translated by the cell machinery, producing the polymerase enzyme. That polymerase then makes more copies of the viral genome. Which in turn gets translated into even more polymerase. Which then make even more copies of the virus... That is where the fork bomb analogy comes in.
That kind of thing doesn't normally happen to us because we don't have those enzymes that can make copies of their own messenger RNA. Putting those fork bombs in production would be asking for trouble :)
Humans have developed protections against some rogue RNA, since out of control fork bombs randomly happening isn't a thing that makes you successful at reproduction.
The Virus reproduces using a fork bomb but for us it's deadly.
