Kaspersky Lab's virus analysts report that they found a piece of malicious adware that was infected with Parite, another virus. They came to the conclusion that the computer used to make the adware was probably infected with Parite. Unlike most other modern viruses, Parite attaches itself to applications on your computers by appending its code to the end of their files.
The answer was simple, and unexpected. When we cleaned the virus from the infected files, we discovered that underneath the Parite infection, the files were infected with three other Trojan-Downloaders - WinAD.c, IstBar.is and Small.aqt, which Kaspersky Anti-Virus has detected for a long time.All of these programs are designed to download adware onto the victim machine. So it seems likely that whoever created the original dropper didn't know that the machine he used was infected with Parite.
On the other hand, it could just be another attempt on the part of virus writers to prevent their creations being detected by dedicated anti-adware and anti-spyware solutions, which can't detect standard file viruses.