The cyber criminals behind ZeroAccess, one of the largest botnets in existence, have lost access to more than a quarter of the infected machines they controlled because of an operation executed by security researchers from Symantec.According to Symantec, the ZeroAccess botnet consists of more than 1.9 million infected computers and is used primarily to perform click fraud and Bitcoin mining in order to generate revenues estimated at tens of millions of dollars per year.