Virus
Definition
The term computer virus is used to describe a computer program designed to destroy, control, or debilitate another computer or its network. Viruses could be used to erase important files or compromise a system's security and ability to process information. Normally, computer users simply say "virus" in reference to a computer virus.
Viruses generally do not destroy hardware themselves, but can disrupt the hardware's ability to communicate to the person using it. Viruses are "spread" through downloaded attachments or computer programs that claim to be something (such as a picture, an important computer update, or game) that they are not. They can also be spread, or infiltrate a computer through the use of a backdoor, which allows a user to remotely install the virus without the user's knowledge.
Viruses as used in the Battlestar Galactica series
The virus infiltration tactic was used greatly in the first Cylon War. In response, the Colonials began using simpler technologies that were impossible to reprogram or shut down. The first battlestars, of which Galactica is a member, used computers but didn't network them or tie them to external sensors or communications, preventing a virus from entering ship's systems and minimizing damage to other systems in the event of an infection. Comparatively primitive communications gear and other hardware throughout the ships also reduced or eliminated the ability of the Cylons to hack into or shut down a shipboard function.
In the Second Cylon War, the Cylons returned to their tactic of infiltration after their espionage determined that Colonial use of computer technology had become widespread once again, 40 years after the first war.
While the Colonial computers were likely resistant to viral infection because they could not be cracked from the outside, after Cylon agents created backdoors in the Command Navigation Program used by almost all Colonial military ships, the attacking Cylon fleet could transmit a signal through the backdoors programmed in the CNP, commanding the CNP to shut down or disable key elements of itself or ship systems. In a sense, instead of trying to break down the Colonial's "door" with a thug (a virus), the Cylons simply changed the door's locks so they could enter by an easier path.
The compromised CNP permits the easy destruction of Colonial defenses and slaughter of countless billions of Colonial citizens living on the Twelve Colonies.
Despite the efforts of decommissioning Galactica and converting it into a museum, the battlestar was still designed to withstand this tactic as this new Cylon war began. While her last squadron of Viper Mark VII's were infected and destroyed early in the initial attack (Miniseries), a squadron of Mark IIs onboard are redeployed. Like their carrier ship, these Vipers are also designed to ward off a Cylon infection or backdoor exploit, and hold their own against future Cylon attacks.
Galactica suffers an Cylon virus infection near the legendary planet of Kobol after doing what Commander William Adama (at the time, seriously wounded in an assassination attempt) would never allow: The temporary networking of several of the battlestar's computers to greatly reduce the computation time needed to find the proper coordinates of the missing civilian Fleet, which Jumped to a different set of coordinates (Scattered). Firewalls (software barriers that deny unauthorized intrusion) are put in place to slow down a Cylon hacking attack, but the protection loses its integrity at the very last moment. After the virus initially shuts down several systems (while, coincidentally, a squad of heavily-armored Cylon Centurions board Galactica), the battlestar uses software or other means to purge the virus (Valley of Darkness).
All appeared successful with the purge until weeks later when the virus returns, this time as a heuristic infection that formed itself as a "Logic bomb", threatening to deny control of the battlestar to its human crew and likely forcing the battlestar's guns on the civilian Fleet and activating the damage control systems to open vents to asphyxiate the crew. The virus is finally removed for good by wiping the hard drives of all computers and restoring them from backups made prior to the Cylon attack (Flight of the Phoenix).
See also: Computer Viruses
Biological Viruses
Of course, viruses are also transmittable diseases of a non-bacterial nature that cause various ailments, such as the Common Cold or the Flu.
See Also
See: Virus