Is Linux Vulnerable to viruses?
In a practical sense, no. Technically...
Due to the design of Linux, it is difficult for viruses to spread far within a system, as they are confined to infecting the user space of the user who executes them. Of course, this is a problem if infected files are launched by root, but as a security conscious individual, you wouldn't be running untrusted files as root, would you?
It is theoretically possible for a virus launched by a regular user to escalate its privileges using system exploits; however, a virus with this capability would be quite sizable, and difficult to write. As of this date, few viruses have actually been discovered for Linux, and the ones that have been discovered aren't worth losing sleep over. This will undoubtedly change with time.
Viruses do exist for Linux, but at the present time are the least significant threat you face. Presently, trojans and worms, which are explained in the following section, pose a greater threat to Linux users.