It's very hard to write a "successful" virus for Linux because of the
built-in security features and as far as I know there has never been
an epidemic of viruses, trojans or worms in Ubuntu. It is possible for
the unwary to click on a link that invokes a javascript routine that
will do damage, but it's still much less lethal than in Windows where
every user has full run of the system files.

Most of the important system files in Ubuntu require root privileges
and only you know the password. I'm no expert and perhaps some of the
senior members can explain more fully.

Linux is a tiny target compared to windows and that is the other
reason that we are usually left alone by the bad guys.