All of these reasons point to viruses as the future of hostile algorithms, however, the most significant reason for this determination is the effectiveness of the virus as a form of attack. Past experiments by Doctor Fred Cohen in 1984 used a normal user account on a Unix system, without privileged access, and gained total security penetration in 30 minutes. Doctor Cohen repeated these results on many versions of Unix, including AT&T Secure Unix and over 20 commercial implementations of Unix. The results have been confirmed by independent researchers worldwide. Separate experiments by Tom Duff in 1989 demonstrated the tenacity of Unix viruses even in the face of disinfectors. The virus used in Mr. Duff's experiment was a simple virus written in script. The virus was believed to have been reintroduced by the operating system from the automated backup and restore system. Re-infection took place after the system had been virus free for one year..