Quick question:

If a clean computer is connected to the same router as the computer that has virus, can the former be infected as well?

This depends on the virus type. Typical Virus will not transfer over network unless you have share drive and have full control on the shared .

 Thanks Kundan.

My PC probably has a virus or two.  My Mac does not, I think.  Both are connected to the same router.

Mac should be relatively immune by the virtue of its better built OS, though the attackes have recently with its rising popularity, I am concerned.  I do run clamx once in a while and so far the result is clean chit.  

Quick question:

If a clean computer is connected to the same router as the computer that has virus, can the former be infected as well?

Absolutely. Don't even need shared drive. Hideous trojans/malwares are constantly searching for open ports/backdoors and guess what ,unprotected network PC could get easily infected if one of your networked PC is infected.

Be sure to have latest:
Antivirus
Antimalwares
and
FIREWALL(Very Important)

Windows Virus probably won't do anything to MAC operating system.

 thanks Bittertruth.

 If your pc is infected with worms, it can infect your mac too.
if you have something critical in your mac, it is better to be safe than sorry.

any worms/virus/malwares written specifically for PC can't infect Macs.

But there are very few mutli-platform hybrid worms that can actually take down any system regardless of whatever OS it's running on. Like I said they're very rare to almost non-existence.

Macs' days of invincibility to virus is ending sooner than you realize.

lol