You need to install clamav or clamd (depending upon your distribution and/or repositories), not clamtk. Clamtk is a toolkit for using clam in user scripts. If you install the clamav/clamd package, it should also install the appropriate cron autoupdate tools for clamav/clamd so the virus definitions will be kept up-to-date.
Linux is pretty much like mac and a few virus rarely target it. There are still anti virus software for linux but less just as what the two above posters have said. You can go to the ClamAv website here. The operating system of linux is pretty much the same as mac thus less virus attacks will occur but that does not mean none. In a few years time, more hackers will develop virus for linux so is better to prepare now and be safe and later be sorry. Good luck