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 e-mail program to spread the virus to other computers, or flood a network with traffic, ultimately making it impossible to perform any Internet activity.