The 456 virus

The 456 virus was a deadly pathogen used by the 456 as a bioweapon. (TV: Children of Earth: Day Four)

Overview

The virus could be released into the atmosphere of a building by a member of the 456 even from within a sealed tank. The virus caused the victims to take on a pale and sweaty appearance before collapsing and dying. The virus had a 100% mortality rate and the time it took to kill a victim appeared to depend on the victim themselves. When initially spread, several people collapsed and died running down stairs, but one man who was in the same room with the alien culprit when it was released survived for a longer period of time before succumbing.

The virus may not be persistent as the bodies of its victims were recovered within hours and not cremated. (TV: Children of Earth: Day Four) Also, the location the virus was released into was habitable again by the next day. (TV: Children of Earth: Day Five)

History

After Captain Jack Harkness threatened a war between the human race and the 456, the 456 ambassador released the virus into Thames House as a display of power. Despite Jack and Ianto Jones' attempts to get the ambassador to release an anti-virus, the ambassador refused. The virus spread throughout Thames House, infecting and killing dozens of people in a short period of time. As Thames House was sealed off when the virus was detected, it did not spread beyond onto Earth. Ultimatly, the only survivors of the virus were Jack due to his immortality and Dekker who got into a hazmat suit in time. The release of the virus finally convinced the human race to give into the 456's demands. (TV: Children of Earth: Day Four)