If your site posses high traffic then hacker can use a program to divert your traffic to another website. Aim is to generate some money from your traffic. Most of the time they use a malware. However Google continuously monitor sites. They may give a warning as follows:
” Google has detected that your site has been hacked by a third party who created malicious content on some of your pages. This critical issue utilizes your sites reputation to show potential visitors unexpected or harmful content on your site or in search results. It also lowers the quality of results for Google Search users. Therefore, we have applied a manual action to your site that will warn users of hacked content when your site appears in search results. To remove this warning, clean up the hacked content, and file a reconsideration request. After we determine that your site no longer has hacked content, we will remove this manual action “
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