No viruses? yes there are viruses...There are not many viruses and a lot are proof of concept but they are there. The rest are macros that attack installed programs again fewer than say what there is for its competition.
Virus.Linux.Alaeda (Kaspersky Lab) is also known as: Linux.Alaeda (Kaspersky Lab), Linux/Alaeda (McAfee), Linux.Alaeda (Symantec), Linux/Alaeda-A (Sophos), Linux/Alaeda.A (Grisoft), Linux.Alaeda.A (SOFTWIN), Linux/Alaeda.A (Panda), Linux/Alaeda.A (Eset)
Alaeda is a non-resident virus. It infects systems running Linux, and is written in Assembler. It infects ELF format files in the current directory.
Virus.Linux.Diesel.962 (Kaspersky Lab) is also known as: Linux.Diesel.962 (Kaspersky Lab), Linux/Diesel.dr.962intd (McAfee), Linux.Diesel (Symantec), Linux/Diesel (Grisoft), Linux/Diesel.B (Panda)
This is a relatively harmless, non-memory resident parasitic virus. It searches for Linux executable files in system directories and subdirectories, then writes itself to the middle of the file. Before searching files, the virus reads its code from the host file. It moves the original bytes to the end of the file and increases the size of the previous section.
As for no Anti Virus programs? there are quite a few even with submit a file features so that htey can be inspected.
http://www.clamav.net/
Here is a list of the main features:
command-line scanner
fast, multi-threaded daemon with support for on-access scanning
milter interface for sendmail
advanced database updater with support for scripted updates and digital signatures
virus scanner C library
on-access scanning (Linux® and FreeBSD®)
virus database updated multiple times per day (see home page for total number of signatures)
built-in support for various archive formats, including Zip, RAR, Tar, Gzip, Bzip2, OLE2, Cabinet, CHM, BinHex, SIS and others
built-in support for almost all mail file formats
built-in support for ELF executables and Portable Executable files compressed with UPX, FSG, Petite, NsPack, wwpack32, MEW, Upack and obfuscated with SUE, Y0da Cryptor and others
built-in support for popular document formats including MS Office and MacOffice files, HTML, RTF and PDF
Viruses do exist in Linux they are mostly Macros though and there are actual and real anti virus software for Linux. There are a ton of proof of concepts....BTW the first major viruses were on UNIX systems in the early 80s