Google uses fully automated systems to scan the Internet for potentially dangerous sites. These systems help detect sites infected with malware and then add a warning that appears in Google search results. The warning is a cautionary page, and we never prevent users from viewing the affected site if they choose. Site owners sometimes say that we’ve made a mistake and that their site does not contain malware. Google’s automated scanners will periodically re-examine the site even if no such request is received, but the process will take longer.”]
Source: https://security.googleblog.com/2009/10/do-machines-dream-of-electric-malware.html