A computer "virus" is a programming code written specifically to copy itself over and over again into other programs. It replicates in such a way that it becomes part of the infected program and is activated every time the targeted program runs. Once running, the virus attempts to contaminate all other programs. Computer viruses can be countered by secure computer software and hardware systems that protect against both intentional and accidental attacks. Virus prevention should focus on both the inside and the outside of a system. Ways of reducing virus vulnerability from the outside include treating public domain and shareware software with caution, not using software from bulletin boards or electronic mail, not connecting to a WAN, using password protected computer dial-up telephones, maintaining a large source of licensed software, and never booting from any floppy disk other than the original write-protected diskette. Ways of reducing vulnerability from the inside include using software programs that identify and prevent viruses from infecting or destroying information and limiting access to the computer and implementing electronic data processing security policies and procedures.