The Cutwail family, also known as Pushdo, is responsible for one of the largest active botnets in the world. Each new variant of the Trojan includes additional features to allow remote attackers access to the infected machine. Trojan.Z also allows it to be automatically upgraded by its master from a remote location over the Internet. Virus researcher Marius Vanta says infections are extremely difficult to spot and should be regularly scanned with a freshly updated antivirus solution. The only visible symptom is increased Internet activity.”]