Geostigma

Geostigma is a disease and a major plot point in the Compilation of Final Fantasy VII.

History and symptoms
Geostigma first emerged around the time Sephiroth was defeated in Final Fantasy VII. The Lifestream helped push Meteor back long enough for Holy to activate and supposedly save the day. However, the Lifestream was previously tainted by Jenova, and everyone that came in contact with it during and since became infected.

In the several years after this event, Geostigma cases continue to rise around the world. No one knows for certain what causes Geostigma, but the prevailing theory is that those close to Mako for extended periods become infected. It is also believed to be contagious, but there is no evidence to back this claim.

Geostigma is the process when a person contracts the Sephiroth Gene, and the immune system tries to fight it. However, the body can't kill the Sephiroth Gene by itself, so the immune system becomes hyperactive and the person develops a disorder similar to Lupus. Symptoms include sudden fatigue, weakness of the body and open sores that secrete black ooze. The disease is almost certainly terminal and there is no known cure up until the events of Final Fantasy VII: Advent Children.