such a simple virus is dangerous because it will quickly use all
available memory and bring the system to a halt. An even more
dangerous type of virus is one capable of transmitting itself
across networks and bypassing security systems. Since 1987, when
a virus infected ARPANET, a large network used by the Defense
Department and many universities, many antivirus programs have
become available. These programs periodically check your computer
system for the best-known types of viruses.
Some people distinguish between general viruses
and worms. A worm is a special type of virus that can replicate
itself and use memory, but cannot attach itself to other programs.