Wildcard coccus

Wildcard coccus (ワイルドカード球菌) is a highly virulent killer bacterium introduced in Shinyaku Toaru Majutsu no Index. It is a bacterium as its name indicates, as such it is of spherical shape when it viewed under the microscope.

It plays a minor role in the Agitate Halation Arc as the final trump card of Tsuchimikado Motoharu against the people who "killed" his sister, Tsuchimikado Maika, though only for threats and negotiations as Motoharu himself didn't want to use it. It was however, set-up for Motoharu to leak it during his rampage, and kill the enemies of the Agitate Halation Project in one fell swoop, as well as Kamijou Touma, though failed as it was killed before it could be used.

There is no mention on what would happen if W. coccus infected a person, other than they would undoubtedly be killed.

Transmission
W. coccus' method of infection was very complex and it would mix in with other microorganisms and multiply. It could be transmitted via air, blood, mouth, or skin contact. It could grow even more dangerous by combining with athlete’s foot, lactobacillus, or other extremely common pathogens.

Prevention
W. coccus is powerful, but it could be easily sterilized by exposing it to powerful ultraviolet light for a certain period of time. That was part of what created such a danger of it being used as a weapon. The case of W. coccus Motoharu carried died after he stopped by a tanning salon, most likely deliberately as he didn't want to use it.