Final Fantasy VII: Advent Children
Geostigma is an incurable illness which appears in the feature film, Final Fantasy VII: Advent Children. At this time in the lifecycle of Final Fantasy VII's universe, Geostigma has become an epidemic across the planet. Vincent explains that the disease is caused when the body is attempting to get rid of Jenova cells, but it overcompensates and causes the disease.
When the Lifesteam was used to destroy Meteor, it was spread out across the planet. Jenova's presence inside the Lifesteam was then exposed to the citizens of the world and people with low immune systems (mainly children) absorbed the cells as a result. The majority of the sufferers were found closer to where Meteor was going to hit though; around Midgar and Edge.
Who Has Geostigma?
In the film we discover that Denzel, Rufus Shinra and Cloud Strife are sufferers of the illness, and that it causes them great pain. Unlike many other citizens, Cloud contracted the disease as a result of Hojo's experiments involving Jenova cells.
Both Denzel and Rufus contracted it normally and Rufus is believed to be in the final stages of the disease. This can be highlighted by the fact he uses a wheelchair, suggesting that he doesn't have long left to live.
Claims Geostigma Is A Gift
Kadaj, one of the Remnants of Sephiroth (Silver Haired Men) lures children who suffer from the disease with the promise of a cure. He informs them that they don't have an affliction, but that they instead have a gift.
This gift is actually Sephiroth's will. Although he was killed by Cloud, Sephiroth remained in the Lifesteam, where he was using Jenova's will to control Geostigma sufferers through the Lifesteam.
When people with the Geostigma disease die, Sephiroth could control the energy when they pass to the Lifestream and eventually control the Lifesteam itself. His plan was to control so much energy that he could use the planet as a vessel to travel through space.
What Cures Geostigma?
During Cloud's fight with Kadaj, he is knocked into the water which has been collecting inside Aerith's Church. When he emerges from the water, he notices that he has been cured of his Geostigma.
The water, which had been mixed with the Lifestream, created a cure for the disease which has plagued so many. Towards the end of the film, Aerith brings forth healing rain from the Lifesteam which cures anyone who comes into contact with it.
Dirge of Cerberus: Final Fantasy VII
Although a cure was found, many people across the planet still suffering from Geostigma during the time of Dirge of Cerberus. When Deepground attack Edge and Kalm in search for pure souls in order to summon Omega, those who were afflicted with Geostigma were ignored or slaughtered in order to obtain those who had never been infected.
They plan to sacrifice the pure souls in order to trick the planet into thinking it was in danger, something which would not have been possible by sacrificing people who were suffering from Geostigma as they were deemed "impure".