Malicious software

From Conservapedia

Malicious software (or malware), often generically called viruses, is computer software which deliberately does something harmful to your computer. The best known kind is the virus, which is designed to spread itself to other computers; that's probably how it got on your computer. Another way malware can get on your computer is to hitch a ride inside another (sometimes legitimate) software package: the trojan horse (or trojan). Unlike the virus, trojans don't spread any further once they get on your computer.

Regardless of how it got on your computer, you now have a problem. The malware may be slowing down your computer or annoying you by sending email to all your contacts. Worse, it may be deleting or copying sensitive information.