I don't recommend anyone bet their life on that assertion. Let's not
forget the RTM Internet Worm of 1988; see:

<http://en.wikipedia.org/wiki/Morris_worm>

It infected and spread among BSD and SunOS systems.

Consider all the security patches we constantly receive for our Linux
distros; those aren't being issued just to keep us busy updating our
systems for the fun of it.

And the nasty DNS problem (cache poisoning) earlier this year most
assuredly hasn't been corrected on all DNS servers yet.

Linux's share of the computer market means that we don't have as much
vulnerability as Windows, but that could change as Linux becomes more
popular.