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.