In fact, virus attacks against Unix systems will eventually become more popular as simpler forms of attack become obsolete. Computer viruses have significantly more virility, methods of protection and opportunity for infection. Methods of protection have been highly refined in viruses, including rapid reproduction by infection, migration through evaluation of its environment, (boot viruses look for uninfected floppy diskettes) armour, stealth and polymorphism. In addition, the host system itself becomes a method of protection and propagation. Virus infected files are protected just as much by the operating system as are non-infected files. Introduction of viruses into systems have also been refined using technology called "droppers". A dropper is a Trojan horse that has a virus or viruses as a payload. Finally, extensive networking technology such as NFS (Network File System) allows viruses to migrate between systems without effort.