What is a virus - computer :: Computer Science

What is a virus - computer What is a VIRUS Virus (computer), self-duplicating computer program that interferes with a computer's hardware or operating system (the basic software that runs the computer). Viruses are designed to duplicate or replicate themselves and to avoid detection. Like any other computer program, a virus must be executed for it to function-that is, it must be located in the computer's memory, and the computer must then follow the virus's instructions. These instructions are called the payload of the virus. The payload may disrupt or change data files, display an irrelevant or unwanted message, or cause the operating system to malfunction. Computer viruses activate when the instructions-or executable code-that run programs are opened. Once a virus is active, it may replicate by various means and tries to infect the computer's files or the operating system. For example, it may copy parts of itself to floppy disks, to the computer's hard drive, into legitimate computer programs, or it may attach itself to e-mail messages and spread across computer networks by infecting other shared drives. Infection is much more frequent in PCs than in professional mainframe systems because programs on PCs are exchanged primarily by means of floppy disks, e-mail, or over unregulated computer networks. Viruses operate, replicate, and deliver their payloads only when they are run. Therefore, if a computer is simply attached to an infected computer network or downloading an infected program, it will not necessarily become infected. Typically a computer user is not likely to knowingly run potentially harmful computer code. However, viruses often trick the computer's operating system or the computer user into running the viral program. Some viruses have the ability to attach themselves to otherwise legitimate programs. This attachment may occur when the legitimate program is created, opened, or modified. When that program is run, so is the virus. Viruses can also reside on portions of the hard disk or floppy disk that load and run the operating system when the computer is started, and such viruses thereby are run automatically. In computer networks, some viruses hide in the software that allows the user to log on (gain access to) Types of VIRUS There are five categories of viruses: parasitic or file viruses, bootstrap sector, multi-partite, macro, and script viruses. Parasitic or file viruses infect executable files or programs in the computer. These files are often identified by the extension .exe in the name of the computer file. File viruses leave the contents of the host program unchanged but attach to the host in such a way that the virus code is run first. These viruses can be either direct-action or resident. A direct-action virus selects one or more programs to infect