We find further suggestion of this concept in the official Final Fantasy VII
novel, "On the Way to a Smile," written by Kazushige Nojima, which serves to
bridge the two-year gap between the events of Final Fantasy VII and those of
Advent Children. In this story, Denzel tells Reeve that those people INFECTED
(note the terminology) with JENOVA's Cells that were spread by the Lifestream
flowing across Midgar were discharging a black pus through the pores of their
bodies. With this in mind, recall that the malady known as "Geostigma" is
revealed in the film to be the result of the body overworking itself in an
effort to rid itself of JENOVA's Cells. When JENOVA's Cells entered people, at
first they would discharge this black pus, as this is the body's natural
reaction to an intrusion by viruses: One's natural immune system involuntarily
rushes white blood cells to the location of an infectious substance within
one's body in an attempt to contain it and then discharge it from the body.
That's exactly what was happening with those infected by JENOVA's Cells.
However, the virus was too strong to be removed as simply as random bacteria
would be.