Viruses do not attack they infect the system.
Routers are mini computers with CPU (many use 486 class of CPU), memory, and some kind of OS. Most of the OS? are propriety, but some (like the Linksys WRT line, use a subset of Linux).
I doubt that the general Viruses that are ?Flying? around can infect a Router since the OS is Not a general OS, and the Router does not Execute (Run) external code.
However, I would not be surprise that a specific Virus can be written to act on a specific Router.
:sun: