When I was a Windows user, I received many times the Virus page in Google. I thought things change in Mac. I received today my first Virus page in Google. I am flabbergasted. The experience raises a question:
How can Google know that there is a virus? How does such a "virus" act? How does it determine to send the Virus page?
I believe Google shows their "virus page" if either you, or in rare cases your ISP, perform far more queries than normal over a certain period of time. I once wrote a little test app to perform certain keyword searches, and after a few thousand searches (within minutes) I began to receive the message. Luckily it went away after a few days.
From the official Google blog:
We maintain a list of such sites through both manual and automated methods. We work with a non-profit called StopBadware.org to come up with criteria for maintaining this list, and to provide simple processes for webmasters to remove their site from the list.
Assuming you mean the "This website may harm your computer"-warning.
Please note that it's not necessarily a 'virus page'. Google also shows that warning if a site is involved with phishing (a fake PayPal or bank login page, for example). It's merely a warning that the contents of the site may be misleading or dangerous.
Having a Mac protects you against almost all viruses in the wild today, but you will still receive that message from Google on bad sites - they display it to everyone, even if you can't get infected.
Simon's answer is generally right.
You can get some more information about methods in Google security blog, e.g.: http://googleonlinesecurity.blogspot.com/2008/02/all-your-iframe-are-point-to-us.html