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A computer virus is a small software program that spreads from one computer to another computer and that interferes with computer operation. Computer viruses spread themselves by infecting executable files, system areas, hard or floppy disks, etc. without the user's knowledge. In addition to this self-replication activity viruses can perform 'bad' actions (payloads) ranging from mild disturbance (annoying the user with silly messages) to provoking damage or outright disaster (e. g. deleting/corrupting files or performing a Denial of Service (DoS) attack). A computer virus is a destructive program that attaches itself to other files and installs itself. Without permission on the computer when the files are opened for use. the virus may cause havoc on the computer system for ax ample, it may delete data on storage devices or interfere with the proper functioning of the computer system.

How Computers infected with a virus
A computer is infected when a copy of the virus resides in the machine. Once the virus is loaded into the memory it can run in background and start to replicate itself. If the infected computer is on the network the infection will propagate very quickly to other machines. This process can be interrupted only by detection and elimination of the virus.
    Types of computer viruses include
  1. Boot sector viruses
  2. They destroy the booting information on storage devises.
  3. File viruses
  4. Attach themselves to files.
  5. Hoax viruses
  6. Come as e-mail with an attractive subject and launches it when e-mail is opened.
  7. Trojans
  8. They appear to perform necessary function functions but perform other undesirable activities in the background without user knowledge.
  9. Worms
  10. Viruses that stick in the computer memory.
  11. Backdoors
  12. May be a Trojan or warm that allows hidden access to a computer system