Apart from their obvious job of providing relevant results, search engines have an equally important one to protect their users from malware ridden sites. These sites may be purely malicious or they may be legitimate sites that have been infected.
Either way, they pose a danger to their visitors. All search engines have a screening process to detect infected sites and either remove them from the results or at least warn users that they may be in danger.
In a study by AV-TEST done over 18 months, Google came out on top. Of the almost 11 million websites tested, only 272 of them had malware, that's 0.0025 percent of them.
Of the 11 million websites tested with Bing, 1.285 had malware, or 0.012 percent, almost five times as many as Google.
Blekko fared better, but it was only tested with three million websites. Of those, 203 were infected. Finally, Yandex had it the worst with 3,330 infected websites out of 13.7 million.