Fred Cohen author of "Computer Viruses: Theory and Experiments" who pioneered ideas in virus propagation and is known for the positive virus(aka compression virus),Cohen's Contradictory Virus,and Cohen's Evolutionary Virus. Cohen began his experiments in 1983 on a VAX 11/750 running UNIX.

"We define a computer virus as a program that can infect other programs by modifying them to include a slightly altered copy of itself. A virus can spread throughout a computer system or network using the authorizations of every user to infect their programs. Every program that is infected can also act as a virus and thus the infection grows."
-Fred Cohen


Basically nothing says they have to be bad, but good viruses only appear on paper not in practice.

-Maestr0