that is how malicious ads work... they appear at basically random... only 1 out of 10,000~20,000 visitors of any site will see them... but that is enough to be effective...

Also, they set cookies (in both HTML cookies and flash cookies) so if you see one... you'll see more b/c they see that you've been "marked" and are vulnerable... (b/c 'really' 1 in 1,000~10,000 visits will see 'infected' ads... but not all infected ads actually 'spring' into being a problem... (b/c they are sneaky that way... at least the 'good ones')....

Moral of the story... Your first/best/last defense against most malware/etc. is an aggressive ad-block that has constantly updated block lists... and wiping ALL cookies regularly (on the order of once 24/hrs, and you can 'keep' a few that you want, but the fewer the better).