There are viruses for linux os you know.
Of course there are. But the magnitude of their ability to spread is extremely low, of course depending on the user's discretion, the privileges of the user account used.
Linux viruses would be actually be very target rich as most web services these days are run on linux and despite few are heard of. The main barrier is file permissions. That just kills the spread vectors.
But all this doesn't mean linux boxes are secure from all attacks. There are vulnerabilities in softwares and one just needs to find a door and a trick to get their way in.