A hidden, self-replicating software program, usually containing malicious logic
that propagates by inserting copies of itself into and becoming part of another
host program. It is designed to infect and gain control over vulnerable systems
without the user's knowledge or consent and is activated when a user runs
or opens its host program. Computer viruses can cause frequent computer crashes
or pop-up messages, corrupt or delete data on a computer, reformat the hard
drive, use an email program to spread the virus to other computers, or flood a
network with traffic, ultimately making it impossible to perform any Internet
activity.