Googles systems have detected a number of bulk subdomain providers becoming targets of abuse by malware distributors. Bulk sub domain providers register a domain name, like example.com, and then sell subdomains of this domain name. Subdomains are often registered by the thousands at one time and are used to distribute malware and fake anti-virus products on the web. In some cases our malware scanners have found more than 50,000 malware domains from a single bulk provider. We offer many services to webmasters to help them fight abuse, such as Webmaster Tools.”]
Source: https://security.googleblog.com/2011/06/protecting-users-from-malware-hosted-on.html