As far as I know, a virus cannot infect the motherboard per se. They have, however, been known to infect the BIOS on the motherboard by overwriting some/all of the code in the BIOS (the code that automatically runs when you turn your system on, and which ultimately starts the loading/running of the OS). It is for this reason that I strongly recommend that the BIOS should be set to read-only mode by means of the jumper and/or BIOS setting (see motherboard manual), unless you are upgrading the BIOS code to a newer revision (and then it should be set back to read-only once the upgrade is complete).