Everyone dreads being the recipient of a computer virus, but not everyone minds studying them. There are researchers who spend a lot of time looking into different types of computer viruses and related security threats in order to determine how they’re programmed, how they do damage, and how they spread. Personally, I find this field interesting, and I enjoy reading about the different types of viruses in existence.

But even if you don’t know much care, basic knowledge about security threats can be useful. It’s sometimes hard to know how a risk must be dealt with before you know its consequences. With a computer virus, however, the consequence is sometimes complete loss of your data or identity theft – so it’s best to learn sooner rather than later!

1. Boot Sector Virus

                                       

The term “boot sector” is a generic name that seems to originally come from MS-DOS but is now applied generally to the boot information used by any operating system. In modern computers this is usually called the “master boot record,” and it is the first sector on a partitioned storage device.

Boot sector viruses became popular because of the use of floppy disks to boot a computer. The widespread usage of the Internet and the death of the floppy has made other means of virus transmission more effective.

2. Browser Hijacker

This type of virus, which can spread itself in numerous ways including voluntary download, effectively hijack certain browser functions, usually in the form of re-directing the user automatically to particular sites. It’s usually assumed that this tactic is designed to increase revenue from web advertisements.

There are a lot of such viruses, and they usually have “search” included somewhere in their description. coolweb may be the most well known example, but others are nearly as common.

3.Direct Action Virus

This type of virus, unlike most, only comes into action when the file containing the virus is executed. The payload is delivered and then the virus essentially becomes dormant – it takes no other action unless an infected file is executed again.

Most viruses do not use the direct action method of reproduction simply because it is not prolific, but viruses of this type have done damage in the past. The vienna virus, which briefly threatened computers in 1988, is one such example of a direct action virus.

4. File Infector Virus

Perhaps the most common type of virus, the file infector takes root in a host file and then begins its operation when the file is executed. The virus may completely overwrite the file that it infects, or may only replace parts of the file, or may not replace anything but instead re-write the file so that the virus is executed rather than the program the user intended.

Although called a “file virus” the definition doesn’t apply to all viruses in all files generally – for example, the macro virus below is not referred to by the file virus. Instead, the definition is usually meant to refer only to viruses which use an executable file format, such as .exe, as their host.

5. Macro Virus

A wide variety of programs, including productivity applications like Microsoft Excel, provide support for Macros – special actions programmed into the document using a specific macro programming language. Unfortunately, this makes it possible for a virus to be hidden inside a seemingly benign document.

Macro viruses very widely in terms of payload. The most well known macro virus is probably Melissa, a Word document supposedly containing the passwords to pornographic websites. The virus also exploited Word’s link to Microsoft Outlook in order to automatically email copies of itself.

6. Multipartite Virus

While some viruses are happy to spread via one method or deliver a single payload, Multipartite viruses want it all. A virus of this type may spread in multiple ways, and it may take different actions on an infected computer depending on variables, such as the operating system installed or the existence of certain files.

7. Polymorphic Virus

Another jack-of-all-trades, the Polymorphic virus actually mutates over time or after every execution, changing the code used to deliver its payload. Alternatively, or in addition, a Polymorphic virus may guard itself with an encryption algorithm that automatically alters itself when certain conditions are met.

The goal of this trickery is evasion. Antivirus programs often find viruses by the specific code used. Obscuring or changing the code of a virus can help it avoid detection.

8. Resident Virus

This broad virus definition applies to any virus that inserts itself into a system’s memory. It then may take any number of actions and run independently of the file that was originally infected.

A resident virus can be compared to a direct payload virus, which does not insert itself into the system’s memory and therefore only takes action when an infected file is executed.

9. Web Scripting Virus

Many websites execute complex code in order to provide interesting content. Displaying online video in your browser, for example, requires the execution of a specific code language that provides both the video itself and the player interface.

Of course, this code can sometimes be exploited, making it possible for a virus to infect a computer or take actions on a computer through a website. Although malicious sites are sometimes created with purposely infected code, many such cases of virus exist because of code inserted into a site without the webmaster’s knowledge.

Conclusion

I’ve included every type of computer virus in this article that I felt was relevant, but this isn’t a list of every type of virus known to man. Also, I did not include any worms or trojans, two common threats that are similar to viruses but technically not the same. If there is a specific type of virus that you feel was missed and is important, let us know in the comments.

What is a virus ?

A computer virus is a program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Viruses can also replicate themselves. All computer viruses are man-made. A simple virus that can make a copy  of itself over and over again is relatively easy to produce. Even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. An even more dangerous type of virus is one capable of transmitting itself across networks and bypassing security  systems.

What do computer viruses do?

Through the course of using the Internet and your computer, you may have come in to contact with computer viruses. Many computer viruses are stopped before they can start, but there is still an ever growing concern as to what do computer viruses do and the list of common computer virus symptoms. A computer virus might corrupt or delete data on your computer, use your email program to spread itself to other computers, or even erase everything on your hard disk.

Computer viruses are often spread by attachments in email messages or instant messaging messages. That is why it is essential that you never open email attachments unless you know who it's from and you are expecting it.

Viruses can be disguised as attachments of funny images, greeting cards, or audio and video files.

The most common virus are :

• Viruses: A virus is a small piece of software that piggybacks on real programs. For example, a virus might attach itself to a program such as a spreadsheet program. Each time the spreadsheet program runs, the virus runs, too, and it has the chance to reproduce (by attaching to other programs) or wreak havoc.
• E-mail viruses: An e-mail virus travels as an attachment to e-mail messages, and usually replicates itself by automatically mailing itself to dozens of people in the victim's e-mail address book. Some e-mail viruses don't even require a double-click -- they launch when you view the infected message in the preview pane of your e-mail software 
• Trojan horses: A Torjan is simply a computer program. The program claims to do one thing (it may claim to be a game) but instead does damage when you run it (it may erase your hard disk). Trojan horses have no way to replicate automatically.
• Worms: A worm is a small piece of software that uses computer networks and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well.

In this article, we will discuss viruses -- from "traditional" viruses to E-mail viruses  and exploits that could target your mobile phone -- so that you can learn how they work and understand how to protect yourself.