You need to understand something about poisoned ads on websites which might infect your computer. Here is the situation in a nutshell: Advertisers do not sell their ads to websites one at a time. Websites that want to make money sell their advertising space to an ad network. Advertisers sign contracts with that ad network which then displays the ads on the participating websites. The ad network sits in the middle between the advertisers and the websites and manages the traffic and the payments.