Virus.Win32.Nsag.a has been causing havoc across the globe for a couple of weeks. The virus infector is a file infector, most likely related to AdWare, and difficult to remove. It seems that several pieces of malware (e.g Trojan-Downloaders) are downloading and/or installing Nsag onto the system. Dedicated anti-spyware solutions cant detect or disinfect infected files, the system is still (partly) infected even after such solutions have been run. This prevents novice users from getting rid of the infection.”]
Source: https://securelist.com/smitfraud-meets-nsag-return-of-the-file-infector/30032/