What is Computer Virus?

Computer virus - A parasitic program written intentionally to enter a computer without the user's permission or knowledge. The word parasitic is used because a virus attaches to files or boot sectors and replicates itself, thus continuing to spread. Though some viruses do little but replicate, others can cause serious damage or affect program and system performance. A virus should never be assumed harmless and left on a system." -- Symantec

Many people are confused about different types of computer virus. The term computer virus is often used broadly to cover several types of malicious programs, including viruses, worms and Trojan horses. Each of them shares some similarities and some subtle differences.
Different types of computer viruses

Computer viruses attach themselves to executable files, and spread from one computer to the other by sharing infected files or email attachments. Computer virus needs a host program to run. The virus codes run once you open the executable files. Some computer viruses can remain hidden in the system and are unleashed at a fixed date and time. Computer virus causes mild to severe damage to boot sector, hardware, computer programs and data files. Depending on their actions, they can be further divided into boot viruses, program viruses, macro viruses etc.

Computer worms replicate themselves in the computer like computer viruses do. But unlike computer viruses, computer worms are self-contained. They can run and spread themselves without being part of a host program. Computer worms spread at enormous speed in the network. Hundreds or thousands of copies can be sent to everyone in your email address book and the process repeats itself in the receiver's computer. Computer worms use up system or network resources quickly, causing the system or network to stop responding. Due to the similarity of computer worms and computer viruses, computer worms are often considered as one type of computer viruses.

Trojan horses are malicious programs in disguise as some useful, innocent programs. This tricks the computer users into running the programs. Sometimes, Trojan horses are hidden codes embedded within a legitimate program. In either case, Trojan horses are hidden applications which are run without the knowledge of the computer users. Trojan horses can damage your files or create security leak in your system, allowing unauthorized users to access your computer. Unlike viruses and worms, they usually do not replicate themselves.

Computer viruses, computer worms and Trojan horses are all malicious programs. Even though they have different modes of replication and action, all of them are extremely destructive.

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