Many new users of Linux think that their system, in contrast to DOS or Windows, is immune to viruses. Rado shows how Linux and Unix systems in general are vulnerable to viruses, too. In fact the first viruses infected Unix systems.
Computer viruses are the most famous members of quite a large collection of electronic beasts.
And, they are the easiest to pick up and infect yourself. Ironically, but just like real life viruses, they are most present where the information infrastructure is high, and virtual hygiene is weak. In fact, there is no OS that is completely safe from viruses. A virus is completely dependent on hosts replication mechanism. Any OS, including Linux.