Both klamav and clamtk are front-ends for the clamAV software. They do check for linux viruses, but they are most useful for making sure your linux computer isn't sharing infected files with windows machines. Most of the virus scanners for linux are actually for servers, and are meant to scan email or uploaded files as they are sent.
Wikipedia has a list of Linux malware, which should help inform about the risks:Linux malware - Wikipedia, the free encyclopedia. They list 30 viruses and various other possible threats.