No, you can't. While Macs are not immune, there ARE no viruses in the wild that run on current Mac OS X.
No, viruses, trojans, worms, etc. are forms of malware. Trojans, spyware, keyloggers, etc. are NOT forms of viruses. There's a difference. Read the link I posted for accurate descriptions of each.
As has been stated, you CANNOT get a virus on your Mac, since none have been created that run on current Macs. You CAN get a trojan, since a few exist, but to do so, you must actively download and install it. Read the link I posted for more details.