So, some crypters and viruses are now detected as hacktools by antiviruses and are of no use now. Bunn has created Fly Crypter which has unique stub generator. So, you can create your own stub and so your crypter becomes your own private version crypter because you can assign and use new stubs to viruses and thus helping your virus program to remain FUD.
How to make Virus undetectable:
Usually, Viruses are detected because their stubs are detected...