Mystique works with Xavier and Forge to stop North Korea from developing and spreading a biological weapon. The virus jumps into a mutant called Host. Host has the power to communicate with viruses and takes the position that the virus has the right to live–just like mutants take that same position when humans say they are too dangerous to live.
Mystique has to track Host across the globe to stop the pathogen, which she does, and she ultimately captures Host who is put in stasis by the “cleaners” working for Forge and Xavier. Their mutant espionage organization is deep, apparently.
It’s a well done spy story–a solid read. In fact, Forge digs the way Mystique does the op so much that he asks her out.