Animus is an organic supercomputer created by Jason Drake.
Animus is able to hijack computer systems. It is also used to control the people who are taken over by it. It makes the person do the command of the person who applied it, and makes the original person's consciousness just an observant passenger. It has a similar effect to the Contessa's voice.
In Rogue, it was used to control Otto Malpense. The strain was still too aggressive for normal people to survive from exposure to it, however the computer inside Otto's own head prevented his death. He could stop the Animus from carrying out certain commands, but it made him weaker every time he did it.
In Zero Hour, the Animus strain has been improved. It has been merged with nanites from Project Panacea, and the Animus now multiplies rapidly when in contact with air. Programmed versions of the Animus make people subject to the will and command of the applier, however unprogrammed versions will simply eat away at anything.