On the lighter side of the 'linux virus' issue here is a link to an old
joke, Click Here <http://www.gnu.org/fun/jokes/evilmalware.html> I
think it helps to get the point across.

You just have to be aware that while the Ubuntu/linux OS is relatively
secure, software like your web browser could still leave you open to
attack like if you give websites permission to download cookies and
such. On the otherside you might do your online banking over an unsecure
wireless network. But using an old army metaphor, if you make it very
hard for others to get in, it is also very hard to get out!

Like others have said, the updates are mainly to fix issues, bugs and
overlooked access rights. The great thing about Ubuntu is that the time
between someone finding an issue and it getting fixed is usually very
quick. Have a look at launchpad just to see the amount of work those
guys put in.