Both spyware and viruses are damaging computer software or files known as malware, but they are spread differently. An important difference between computer spyware and a virus is that spyware snoops into computer activities, often sending information about the user to a third party, while a virus does not. Another vital difference is that unlike spyware, a virus often makes copies of itself and tries to use one computer to spread the virus to others. Although both can affect a computer's operation negatively, spyware seldom seeks to harm the system, while viruses often cause a great deal more lasting damage.
Spyware and a virus are both usually loaded onto a computer without the user being aware and run without the user's knowledge or consent. The typical mode of delivery for spyware is in a bundle with other files or programs that the computer user chooses to download. Spyware sneaks on to the user's system along with the desired material. Viruses, however, are most often spread as attachments to e-mail or through instant messages, although they can also be hidden in downloaded material.
Although they can both harm a computer, another difference between spyware and a virus is found in the primary purpose of the malware. Spyware is designed with the purpose of recording a computer user's activities and relaying that information to a third party. It can track a person's computer usage habits and transmit the information to advertisers and others. Another, more dangerous, use can steal private information like e-mail addresses, passwords, credit card numbers, and other important personal data and transmit it to a hacker or identity thief.
The primary purpose of a virus is to copy and spread itself. It will often replicate itself multiple times, thereby using up the computer's resources and causing the system to fail. Another common virus tactic is to access a computer user's e-mail address book and send copies of itself to all of the addresses. It may also try to use other methods of transmission where one computer communicates with another such as in file sharing services and instant messaging. Spyware does not make copies of itself or transmit itself to other computers using the infected system.
Another difference between spyware and a virus involves the intent of the author. The spyware author usually wants to gather information which will be used for various purposes. Spyware needs an operational system to accomplish this so it rarely harms the computer, although it uses the computer's resources and can make it run slower. Viruses often are written with more destructive intentions, and often make computer systems and networks unusable. After they've spread themselves, viruses frequently cause lasting damage to the operating system and hardware.