Drura Sehpt comes from the planet Somahtur, a little known, perpetually quarantined world. Somahturians' bodies act as hosts to vast colonies of bacterial and viral microorganisms, while remaining immune to their effects. They also have the ability to transmit them as diseases, and at least in Drura's case, combine/form them into any new disease she has encountered.
Thanks to Scars of Mirrodin for the Infect and Proliferate keywords! They seem to be tailor-made for her. Immune is a keyword that I made up for her ability.