What Is a Computer Virus?

Virus warning on computer screen
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'Virus' is an umbrella term used to describe unwanted malicious programs that install themselves onto your computer. Viruses will cause you a range of damage, from the very mild to the entire loss of your computer data. A good way to describe viruses is to call them malware or software programs that have malicious intent.

Viruses/malware are commonly broken down into four categories:

Classic Virus

Classic virus is a term that was coined in 1983. These viruses are malicious programs that rewrite existing computer code on your computer. Classic viruses are not so much unwanted additions to your system as they are mutations of existing code.


Trojans, or Trojan Horses, are additions to your system. These malicious programs masquerade as legitimate files in your email, deceiving you into willfully adding them to your hard drive. Trojans rely on you to intentionally open your computer to them. Once on your machine, Trojans then function as independent programs that operate secretly. Trojans commonly steal passwords or perform denial of service (overload your system) attacks. Examples of Trojans include Backdoor and Nuker.


Worms, or Internet Worms, are also unwanted additions to your system. Worms are different from Trojans, though, because they copy themselves without your direct assistance... they robotically worm their way into your email, and begin broadcasting copies of themselves without permission. Because they do not require user intervention to reproduce, worms reproduce at an alarming rate. Examples of worms include Scalper, SoBig, and Swen.

Adware and Spyware

Adware and Spyware are cousins to trojans, worms, and viruses. These programs lurk on your machine. Adware and spyware are designed to observe your Internet habits and then pummel you with advertising, or to report back to their owners via secret messages. Sometimes, these products will even use your hard drive to store and broadcast pornography and advertising back to the Internet.

These semantics and definitions of viruses/malware can be very obscure to the non-technical user. However, it isn't crucial to distinguish between these products technically. What is important is how you consciously defend against these malware infections.