Computer viruses are software programs that invade a computer system to wreck havoc with computer files or the computer users themselves. Some are benign causing nothing more than a nuisance while others can delete files or even destroy computer hardware. Typically viruses are programmed to reproduce themselves and spread from one computer system to another, just like a normal human virus spreads via human contact throughout the population. Computer viruses can be spread via diskettes or other removable media, by way of email attachments, or in some cases simply by being connected to the Internet and not protected.
Type Of Computer Viruses
Computer viruses are categorized into four main types: boot sector, file or program, macro, and multipartite viruses.
Boot sector viruses are usually transmitted when an infected floppy disk is left in the drive and the system is rebooted. The virus is read from the infected boot sector of the floppy disk and transferred to the master boot record of the system's hard drive. Then, whenever the computer is booted up, the virus will be loaded into the system's memory. Any floppy disk formatted as a bootable disk in the infected machine would contain the virus and spread itself to other machines.
Program or file viruses are software programs that attach themselves to executable programs. Once the infected program is run, the virus is transferred to your system's memory and may replicate itself further. It may spread to all the files on a hard drive or to files of a specific type. Common viruses such as Happy99, PrettyPark,and the ILOVEYOU worm can be classified as this type.
Macro viruses are currently the most commonly found viruses. They infect files run by application that use macro languages, like Microsoft Word or Excel. The virus looks like a macro in the file, and when the file is opened, the virus can execute commands understood by the application's macro language. The infamous Melissa virus is a macro virus as well as the common Laroux Excel spreadsheet virus.
Multipartite viruses have characteristics of both boot sector viruses and file viruses. They may start out in the boot sector and spread to applications, or vice versa
