From LQWiki

A virus is a malicious program that "infects" computer systems and replicates itself by infecting applications and/or the boot sector of floppy disks inserted into the system.

Many worm and virus programs are released to "capture" computers for the use of the worm/virus author. The computers are typically used to send SPAM and to launch Denial of Service attacks on other systems.

Traditional program-infecting viruses are very rare on Linux, due to the in-built security that prevents ordinary users from modifying system executables. Vulnerabilities in Linux tend to fall into the categories of Trojan horse programs, worms, rootkits, and local exploits that allow ordinary users to gain root privileges.

Consequently, virus scanning software is less common for Linux. Instead, there are tools that can search for rootkits. Further, Linux software occasionally needs to be patched. Any new software packages or patches should have its md5sum or gpg signature validated before installation in order to help ensure that a package has not been tampered with.