A virus is any type of software that embeds itself into your computer and can then replicate itself by infecting other computers through a network, Internet connection or removable media such as CD, usb thumb drive or floppy disk. Though the term is often confused or mistakenly used to mean other types of spyware or general malware, a virus is malicious software that actually copies itself to other computers and infects them.
Though they can be transmitted by the physcial "sneaker net" (carried from pc to pc via disk or USB drive), they are most often transmitted via e-mail.
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Though by definition a virus must have a reproductive function, it is often used to refer to other types of malware such as trojan horses, rookits, etc. that do not replicate themselves. A host computer must first be infected, possibly by a regular, manual hack attempt, or it could be infected by payload carried by another application such as a trojan horse or other spyware.
A virus only needs one host computer to start its infestation process to other computers, since replication is one of the main functions. A host PC is infected, and then the virus may send itself out to every person on the host computer's email address book. Every recipient of the email who doesn't use virus protection is at risk of infection. Those that are newly infected in turn become new virus hosts, which then replicate to more computers, again possibly found from the new host's email address book. The cycle then continues.
We suggest using a capable virus scanner and remover regularly. These applications can even run in the background to alert you of any incoming virus before it has the chance to deploy its infection or replication process. If you suspect you have a virus on your computer, get started with our step-by-step spyware remover software guide right away.