In computer security, computer virus is a self-replicating computer program that spreads by inserting copies of itself into other executable code or documents. A computer virus behaves in a way similar to a biological virus, which spreads by inserting itself into living cells. Extending the analogy, the insertion of a virus into the program is termed as an "infection", and the infected file, or executable code that is not part of a file, is called a "host". Viruses are one of the several types of malicious software or malware. In common parlance, the term virus is often extended to refer to worms, trojan horses and other sorts of malware; viruses in the narrow sense of the word are less common than they used to be, compared to other forms of malware. However, in a stricter sense, viruses, worms and Trojans are different from one another. They have different characteristics and behaviors.

While viruses can be intentionally destructive, for example, by destroying data, many other viruses are benign or merely annoying. Some viruses have a delayed payload, which is sometimes called a bomb. For example, a virus might display a message on a specific day or wait until it has infected a certain number of hosts. A time bomb occurs during a particular date or time, and a logic bomb occurs when the user of a computer takes an action that triggers the bomb. The predominant negative effect of viruses is their uncontrolled self-reproduction, which wastes or overwhelms computer resources.

Today, viruses are somewhat less common than network-borne worms, due to the popularity of the Internet. Anti-virus software, originally designed to protect computers from viruses, has in turn expanded to cover worms and other threats such as spyware, identity theft and adware.

Scanning Your Computer for Viruses using Online Virus Scanning Services

Included in the many types of viruses are:

Trojan horses
A Trojan horse is just a computer program. The program pretends to do one thing (like claim to be a picture) but actually does damage when one starts it (it can completely erase one's files). Trojan horses cannot replicate automatically.

Worms
A worm is a piece of software that uses computer networks and security flaws to create copies of itself. A copy of the worm will scan the network for any other machine that has a specific security flaw. It replicates itself to the new machine using the security flaw, and then begins scanning and replicating anew.

E-mail viruses
An e-mail virus will use an e-mail message as a mode of transport, and usually will copy itself by automatically mailing itself to hundreds of people in the victim's address book.

Logic Bombs
Logic bombs maliciously cause legitimate applications to fail. "An application, for example, might delete itself, along with all information and files from the disk, after a couple of runs as a copy protection scheme. The logic bombs may transfer itself to other applications and storage items, i.e. floppy disks, CD-ROM disks, and CD Disks."

A computer virus will pass from one computer to another like a real life biological virus passes from person to person. For example, it is estimated by experts that the Mydoom worm infected a quarter-million computers in a single day in January of 2004. In March of 1999, the Melissa virus spread so rapidly that it forced Microsoft and a number of other very large companies to completely turn off their e-mail systems until the virus could be dealt with. Another example is the ILOVEYOU virus, which occurred in 2000 and had a similarly disastrous effect.

A computer virus is a small program written to alter the way a computer operates, without the permission or knowledge of the user. A virus must meet two criteria:

It must execute itself. It will often place its own code in the path of execution of another program.

It must replicate itself. For example, it may replace other executable files with a copy of the virus-infected file. Viruses can infect desktop computers and network servers alike.
Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply to replicate themselves and make their presence known by presenting text, video, and audio messages. Even these benign viruses can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.

Trojan Horses are imposters--files that claim to be something desirable but, in fact, are malicious. A very important distinction between Trojan horse programs and true viruses is that they do not replicate themselves. Trojans contain malicious code that when triggered cause loss, or even theft of data. For a Trojan horse to spread, you must invite these programs onto your computers--for example, by opening an email attachment or downloading and running a file from the Internet. Trojan.Vundo is a Trojan.

Worms are programs that replicate themselves from system to system without the use of a host file. This is in contrast to viruses, which requires the spreading of an infected host file. Although worms generally exist inside of other files, often Word or Excel documents, there is a difference between how worms and viruses use the host file. Usually the worm will release a document that already has the "worm" macro inside the document. The entire document will travel from computer to computer, so the entire document should be considered the worm W32.Mydoom.AX@mm is an example of a worm.

How to Protect Yourself

Use anti-virus software
Install anti-virus software on your computer, and ensure that it is kept up to date.  Because new viruses can spread extremely quickly, it is important to update the virus pattern files on a regular basis.  Old anti-virus software will probably do little to protect you from new viruses.

Use an email filtering solution
Since most viruses are spread through email, it is important to use an email system that scan inbound and outbound email and blocks viruses.  Most popular antivirus software does this however find out if your service provider blocks viruses at the email server level.  Some service providers charge a fee for this but in the end, it is worth the extra cost. 

Use a Firewall
Use a firewall to protect your computer or computer network from the internet.  Computers connected to the outside world, such as those connected directly to a DSL modem, a cable modem, or a dialup connection, should be properly protected from internet threats via a firewall.  The firewall can be either a piece of hardware separating your computer or network from the outside world, or the firewall can be a software type firewall, such as the firewall built into Windows XP.

Stay up-to-date with software patches
Many software vendors issue advisories on security issues. Turn on Automatic Updates in Windows and install updates.

Back up your data regularly
Make regular backups of important data and files like pictures and documents. If backing up to tape drives, CDs or DVDs, check that the backups were successful. You should also find a safe place to store your back-ups, perhaps even off-site in case of fire.

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