Quote Originally Posted by Squall of SeeD
"Geostigma" is the name given to the side-effects of an internal struggle
between JENOVA's Cells and one's own Spirit Energy, gifted to them at
conception by the Lifestream. These side-effects result from the Spirit Energy
causing one's body to basically over-tax itself in trying to expel JENOVA's
Cells from within. These side-effects consist of extreme fatigue, open sores
on the surface of the skin, and discolouration of the skin. It is actually
these negative side-effects that contribute to an individual's death rather
than the presence of JENOVA Cells alone.
People became infected with Jenova's Cells when its cells were spread across the planet accidentally by the Lifestream at the end of FF7. Avalanche had destroyed Jenova near the center of the planet on their way to battle Sephiroth.