Viruses and worms are characterized by the ability to make and spread
copies of themselves; the difference between them is that a worm is a
complete program that replicates itself, while a virus is a bit of code
that is inserted (and inserts copies of itself ) into another piece of software
or a file, so that it is executed as part of the execution of that piece
of software or as a result of opening the file. Viruses and worms typically
cause problems such as consuming network bandwidth as mere
side effects of attempting to spread copies of themselves. Even worse,
they can also deliberately damage a system or undermine its security in
various ways. They could, for example, install a backdoor—software that
allows remote access to the system without the normal authentication.
This could lead to a firewall exposing a service that should be providing
its own authentication procedures but has been undermined by a
backdoor