Types Of Computer Viruses

A computer virus is a program that is designed to damage your computer, programs, and files. Like a virus in a living thing, a computer virus can spread if it is not removed. Some viruses are more dangerous than others. One of the most common places for a computer virus to appear is on a file found on the Internet or attached to an E-mail. For example, you may have a virus that just pops a message box on your screen, and then the virus is disabled, or you can have a virus that deletes half of your hard drive.
Computer viruses didnít really exist until the mid 1980s. The first computer viruses were created in university labs to demonstrate how much of a threat the vicious code could be.
There are many kind of viruses that exist today. Some of the most common of Computer Viruses are: File Viruses, Boot Sector/Partition Viruses, Multi-Partite Viruses, Trojan Horses, File Overwriters, Polymorphic viruses, and Stealth Viruses.
A File Virus is the most common kind of virus. These kinds of viruses usually infect .EXE and .COM files, which are the main component of a program or application. A file virus can insert its own code into part of the file, so that when the infected program file is run, the virus is executed first.
Most file viruses are memory resident. Because of this, they can easily attach themselves to other programs that are being run and start to infect that file. A simple virus will overwrite and destroy a host file, immediately letting the user know that there is a problem because the software will not run. Because these viruses are immediately sensed by the computer, they have a less chance to spread. More complex written viruses will cause more damage, spread easier, and are harder to be detected.
Boot sector viruses infect hard drives and floppy disks by putting itself on the boot sector of the disk, which has the code that is run at boot up. Booting up from an infected floppy, allows the virus to jump from the floppy to the hard drive. These viruses are loaded first, and gain control of the system before MS-DOS could be loaded. Since the virus is run before the operating system, it is not MS-DOS-specific and can infect any PC operating system.
These viruses stay in the RAM and infects every disk that is read by the computer until the computer is rebooted. After reboot the virus is removed from memory.
Multi-Partite Viruses are the worse of both file and boot sector viruses. They can infect the host software components. These viruses spread like a file virus, but still insert itself into a boot sector or partition table. Because of this, they are difficult to remove. An example of this type of virus is the Tequila virus.
Trojan Horses are the worst kind of viruses that exist. They contain malicious code that is meant to damage your computer. Unlike other viruses, this virus does not replicate itself. This virus waits until the trigger event. When the trigger event occurs, a message is displayed or files are damaged. Because of the nature of these viruses, there are some researchers that do not classify Trojan Horses as viruses.
File Overwriters are viruses that link themselves to a program, leaving the original code intact and adding themselves over and over to as many files as possible. These viruses are made simply to keep copying itself. While, this virus is copying itself it is taking up more disk space and slowing down performance. Since these viruses often have flaws in them, they can inadvertently damage or destroy data. The worst kind of file overwriters, wait until the trigger event, then start to destroy files.
Most of the viruses that exist today are Polymorphic. Recently a Mutation Engine was released. This software ensures that polymorphic viruses will only proliferate over the next few years. Like the human AIDS virus, polymorphic viruses grows fast to escape detection by anti-virus programs. Special encrypted code within this virus allows the virus to hide from detection.
There are a limited number of kinds of polymorphic viruses. Because of this, they are easier to notice. An example of this would be the Whale Virus , which