Types of viruses

There are thousands of viruses today. More and more viruses are discovered nowadays. So its becoming difficult to detect and destroy new viruses. The new viruses are programmed in such a way that they can enter in to the computer memory without detecting by the anti viruses. So the anti virus companies are stepping up the security levels. There are different types of viruses nowadays. Some of them are given below.

• File viruses (Parasitic Viruses)

File viruses or parasitic viruses are a piece of code or application that is attached themselves to the other files that are executable or driver files or compressed files. They get activated when the host program is executed. After activation these viruses start spreading by latching themselves to many other files and thus they spread like a forest fire. Then they start destruction to the data or loss of files or corruption of files. Most of the viruses of this type when activated enters in to the computer memory and searches for the other files which can be infected by them. It can even spread and infect the other systems that are shared with it.

Besides spreading themselves these viruses perform destructive activities also. The destructive activity can be activated by means of a 'trigger'. The trigger may be the execution of the host file or the virus file by itself, otherwise the trigger may be some date or time. The date and time can be obtained from the system date and time. The trigger may be the number of times the virus has replicated or something similar to it. The examples of file viruses are: Randex, Meve, MrKlunky, Casino, Boza, Tentacle, Win32/CIH.

• Boot Sector Viruses

They are also known as System Sector Viruses. Boot Sector Viruses infects the boot sector which is a crucial part of a computer system. The boot sector is where all information about the drive is stored, along with a program that helps the virus in loading into memory at the time of every booting. The Boot Strap Virus does not affects the files. First it moves or overwrites the original boot code, replacing it with infected boot codes. Then the virus will move the original boot sector information to another sector on the disk, marking that sector as a bad spot on the disk so it will not be used in the future. To be infected by this type of virus, you must boot the computer using an infected floppy disk. For example, if a user leaves an infected floppy disk in the disk drive and you reboot the computer, then you will bring the virus into the system. The inability to attack the files leads to their downfall. In the era when floppies where used these viruses spread like a wild fire. But the introduction of CDs reduced their spreading. However some of them still exists. The operating systems of today prevent them from activating. Examples of Boot Sector Viruses: Joshi, Devil's Dance, V-Sign, Polyboot.B, AntiEXE.

• Multi-Polymorphic Viruses

This type of virus affects both boot sectors and executable files. They can combine some of the characteristics of stealth and polymorphic viruses. These viruses spread through infected media and reside in the memory. They then move to the boot sector of the memory. From there it infect the executable files in the system and it spread across the system. Today also there are many multi-polymorphic viruses in existance. Example of muli-polimorphic virus is Ywinz.

• Macro Viruses

These kinds of viruses use an application's own macro programming language to distribute themselves. Macro viruses can infect Word files, as well as any other application that uses a programming language. These viruses infect documents, templates but not programs. When you open a document or a template that contains a macro virus, then the virus will spread to other documents and templates you may have on your system. For example, a macro virus can change, delete document contents, change settings in the Word environment, set a password, copy a DOS virus to the user's system and much more… Moreover, macro viruses have the potentiality of spreading across different platforms such as PC to Mac. Because they are programmed to work with the application than with the operating system. This makes them platform independent. If you are familiar with the Word macros you have on your system, you can look through the various macros for ones that you do not recognize. The first macro virus was written for Microsoft Word and was discovered back in August 1995. Today there are thousands of macro viruses exists. Examples of types of macro viruses: AAAZAO, AAAZFS, AutoOpen, FileSaveAs, PayLoad, Relax, Melissa.A, Bablas etc.

For more information about Macro Viruses see http://www.bu.edu/computing/virus/macro-protection.html

• Network Viruses

These viruses are capable of fast spreading through networks including LAN and internet. It is commonly transfered through shared drives and folders. Once it affects a system it searches for other vulerable systems and infects it. Examples of the Network viruses are: Nimda, SQLSlammer.

• E-mail Viruses

These viruses are a form of macro virus that spreads itself to all the contacts in the address book. If any of the e-mail recipients opens the attachment of the infected mail, it spreads to the address book of the recipient and thus they spreads like a wild fire. Nowadays viruses are capable of infecting the system even if the infected mail is previewed in a window. Example of the e-mail viruses: ILOVEYOU virus