The Soeki Soeki no Mi, or Plague Plague fruit was a disease based Paramecia devil fruit devoured by "Black Plague" Jackson.
This Devil Fruit allowed the user immunity to all diseases though they seemed to affect the users body in some way as Jackson appeared gaunt and very near death all the time. In turn for near immortality the user can generate any disease in existence or bioengineer new one by cross fusing ones in existence. Diseases can then be spread by touch, breath, blood and worst of all, by his hypodermic needle like fingers. In a way this fruit is the opposite of the Horm Horm fruit. The user of this fruit fought agains the No Chance 9 long ago and was defeated and killed, but not until after he killed one of their teammates aswell beginning the downward spiral leading to them leaving into the new world. It seams however that Devil fruit users have an unnatural resistance to this fruit disease based attacks as those with Devil Fruits are able to survive longer and resist longer. This is potentially the most dangerous fruit in existence as not even darkness can drag down disease.