If any software can be written for any given computer system, it cannot be entirely virus/trojan/malware free. There isn't much out for Macs right now since most computer users use Windows, and malware writers will target the system where they can cause the most damage.
No. It is not virus free. It actually is very dissicult to give your Mac a virus. Also Linux doesn't get viruses that easily. Unlike Mac and Linux, Windows and Android are vulnerable to viruses.