Virus and Anti_virus


In computers, a virus is a program or programming code that replicates by being copied or initiating its copying to another program, computer boot sector or document. Viruses can be transmitted as attachments to an e-mail note or in a downloaded file, or be present on a diskette or CD.
Generally, there are three main classes of viruses:
File infectors. Some file infector viruses attach themselves to program files, usually selected .COM or .EXE files. Some can infect any program for which execution is requested, including .SYS, .OVL, .PRG, and .MNU files. When the program is loaded, the virus is loaded as well. Other file infector viruses arrive as wholly-contained programs or scripts sent as an attachment to an e-mail note.
System or boot-record infectors. These viruses infect executable code found in certain system areas on a disk. They attach to the DOS boot sector on diskettes or the Master Boot Record on hard disks. A typical scenario (familiar to the author) is to receive a diskette from an innocent source that contains a boot disk virus. When your operating system is running, files on the diskette can be read without triggering the boot disk virus. However, if you leave the diskette in the drive, and then turn the computer off or reload the operating system, the computer will look first in your A drive, find the diskette with its boot disk virus, load it, and make it temporarily impossible to use your hard disk.
Macro viruses. These are among the most common viruses, and they tend to do the least damage. Macro viruses infect your Microsoft Word application and typically insert unwanted words or phrases.
Definition: "antivirus" is protective software designed to defend your computer against malicious software. Malicious software, or "malware" includes: viruses, Trojans, keyloggers, hijackers, dialers, and other code that vandalizes or steals your computer contents. In order to be an effective defense, your antivirus software needs to run in the background at all times, and should be kept updated so it recognizes new versions of malicious software. Brain virus

Early Examples

Brain virus

The first computer virus for Microsoft DOS was apparently written in 1986 and contains unencrypted text with the name, address, and telephone number of Brain Computer Services, a store in Lahore, Pakistan. This virus infected the boot sector of 5¼ inch floppy diskettes with a 360 kbyte capacity. Robert Slade, an expert on computer viruses, believes the Brain virus was written as a form of advertising for the store in Pakistan.

Lehigh Virus

In November 1987, a virus was discovered infecting the COMMAND.COM file on DOS diskettes at Lehigh University. When an infected COMMAND.COM had infected four other copies of COMMAND.COM (i.e., when copying to a floppy diskette), the virus wrote over the file allocation table on all disks in the system, destroying the ability to read files from those disks.

Christma Worm

A student at a university in Germany created a worm in the REXX language. He released his worm in December 1987 on a network of IBM mainframe computers in Europe.

The worm displayed an image of a conifer tree on the user's monitor, while it searched two files on the user's account to collect e-mail addresses, then automatically sent itself to all of those addresses.

Morris Worm

On 2 November 1988, Robert Tappan Morris, then a first-year graduate student in computer science at Cornell University, released his worm that effectively shut down the Internet for several days.

MBDF Virus

In 1992, four undergraduate students at Cornell University created and released the MBDF virus, which attacks Apple Macintosh computers. This virus was released in three shareware programs:

Pathogen Virus

In April 1994, the Pathogen computer virus was released in the United Kingdom, by uploading an infected file to a computer bulletin board, where victims could download a copy of the file.

Melissa Virus

The Melissa virus was released on 26 March 1999 and was designed to infect macros in word processing documents used by the Microsoft Word 97 and Word 2000 programs. Macro viruses were not new, they had been known since 1995.