One of our clients recently was impacted by CryptoWall. It's nasty.
Security researchers at Proofpoint warn that a new variant of CryptoWall recently spread through malicious banner ads. Surfers ran a risk of being faced with ransomware purely by visiting one of the impacted sites, which included various properties in the Yahoo!, Match.com, and AOL domains, among others.comment makes a good point that the ad networks should be called out for their participation.
This "drive-by" risk is very difficult to protect against. I believe that an ad-blocker will help mitigate this risk. I use Adblock Plus.
Lifehacker had a good article on all the things you can do with Adblock Plus. I also found that Adblock Plus has a page that will let you extend it.
Here're my options:
Adblock Plus is not without controversy. Read about it here.