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 on different levels of the building, but Ianto Jones who was in the same room with the alien culprit when it was released survived for a longer period of time before succumbing.

The bodies of its victims did not appear to be contagious after death as they were recovered within hours and not cremated. (TV: Children of Earth: Day Four) The virus itself could disperse after agents cleared the building of the toxin, making the building it was released into habitable again by the next day. (TV: Children of Earth: Day Five)

History
After Captain Jack Harkness threatened to declare war between the human race and the 456, the 456 ambassador released the virus into Thames House as a threat. Despite Jack and Ianto Jones' attempts to force the ambassador to release an anti-virus, the ambassador refused. The virus spread throughout Thames House, infecting and killing dozens of people in minutes. As Thames House was sealed off when the virus was detected, it did not spread beyond onto Earth. Ultimately, 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 convinced the human race to give into the 456's demands. (TV: Children of Earth: Day Four)