People are mixing their terms, viruses, malware, rootkits, exploits are all different things.

There are plenty of exploits available for all OS, there are plenty of rootkits too, viruses "tend" to effect Windows more than anything, as of this time there are no reported "viruses" in the wild for Linux and if there was it would get patched pretty quickly, as for malware that is a contraction of malicious and software and encompasses all of it, though exploits dont tend to come under that banner though an exploit might be the result of a virus.

Linux exploits (which are not viruses) number in their hundreds typically and upto the thousands across the board

Just to clear things up