Tony StillI have just encountered a pretend-virus page on my Mac for the second time this week so I guess it's quite common.
Today, I clicked an apparently legitimate search result (about a Steve Jobs' quote), in the first page of results, from Google that was supposedly going to take me to denofgeek.com (that I believe to be reputable). Instead I ended up on page with an Apple logo that beeped at me and opened a pop-up saying "IMMEDIATE ACTION REQUIRED, We have detected a trojan virus (e.tre456_worm_osx) on your Mac." against a background of alleged further infections and other scary nonsense.
The web address starts with http://safety.apple.com but continues with
.lrnozelo.qwpj1i3dzpn87ci478q7cmygedorbtd.stream and a load of parameters.
This is, of course, a scam. Don't click the suggested OK button (that doesn't appear), nor the provided Continue button. Just close the page and allow yourself to feel a moment of superiority over some half-wit criminal out to trap the unwary.