The Stigma. It's a symptom of alien matter infesting the body. The body tries to eliminate it, and overcompensates. Inside our bodies is a current like the Lifestream. That current is what fights off any malevolent intruders.Vincent Valentine
Geostigma (星痕症候群, Seikon Shōkōgun?, lit. Planet Scar Syndrome), also known as The Sephiroth Gene and Jenova's Memetic Legacy, is a disease in Final Fantasy VII: Advent Children. In the ending cutscene of Final Fantasy VII the lifestream sweeps throughout the planet, particularly in Midgar, but this has the unexpected side-effect of infecting the world with the plague.
Before a proper name was applied to the epidemic, Geostigma was known as the Midgar pox (ミッドガル病, Middogaru yamai?) by those not living in Midgar.
Cause and symptoms
When Meteor began to fall on Midgar at the end of Final Fantasy VII, the lifestream emerged to push it back. Because some parts of the lifestream had been infected with Jenova's memetic legacy, the people who came in contact with this tainted lifestream contracted Geostigma. The disease is not contagious, but is believed to be so.
In Final Fantasy VII: Advent Children, Vincent Valentine says Geostigma is caused by the host being infected with the Sephiroth Gene. in On the Way to a Smile, the novella "Lifestream: Black", Sephiroth describes traveling the world in the lifestream, leaving his mark on those with vulnerable spirits, which in turn spreads despair and leaves more people open to infection. In Advent Children Complete, Rufus Shinra states the public believes mako energy to be the cause of the disease.
When someone is infected with Geostigma they contract the Sephiroth gene, and the immune system works to try to eliminate it. This fails to remove it, and the immune system overcompensates fighting an incurable contamination. Those infected with Geostigma experience debilitating weakness, and develop open sores on their body that secrete black ooze. Some sufferers experience hallucinations and seizures.
Although its severity varies from person to person, Geostigma is invariably fatal. The disease has no known cure. Cloud was searching for one, and as noted by Barret in On the Way to a Smile, even if a cure were to be found it would be next to impossible to deliver and administer it to all those infected around the world.
Because their immune systems are weaker, a great number of children around the world have been infected. The infection is most concentrated in Edge, a town populated by the Midgar survivors that rests on the Midgar ruins' outskirts. Three of the major characters, Cloud Strife, Denzel, and Rufus Shinra, carry the disease. As a result of so many dying from the disease, the citizens are used to sights of death.
Kadaj, one of the remnants of Sephiroth, lures children afflicted with Geostigma to the Forgotten City with the promises of curing them. He has them drink from a pool of water tainted with Sephiroth's will and negative lifestream, placing them under his control. The Sephiroth gene within the children have their "reunion instinct" awakened, and lead Kadaj and his brothers to Edge to find Jenova.
After assimilating Jenova's remains into himself, Kadaj becomes a host body for Sephiroth, who is reborn. Sephiroth reveals he was using Jenova's will to control those with Geostigma through the lifestream and that eventually enough infected people would die of the disease allowing him to control the lifestream and take control of the planet. He would use the planet as a vessel to travel the universe and attack another planet, as Jenova herself had done for eons.
Cloud kills Sephiroth and the other remnants of Sephiroth are felled as well. The spirit of Aerith Gainsborough brings down healing rain from the lifestream, which cures everyone in contact with it of their Geostigma. Others can go to Aerith's church where a spring of water infused with pure energy from the lifestream cures anyone infected with the disease.
A year later, the mysterious group known as Deepground attacks Edge and Kalm abducting those who were never afflicted with Geostigma and either ignore, or more likely slaughter, those who were, being considered "tainted" for their plans. Deepground is trying to trick the planet into thinking it is in danger by filling the lifestream with untainted spirits to summon Omega. Those who had Geostigma were considered "impure," and could not be used to trick the planet.
Behind the scenes
According to the Reunion Files for Advent Children, the concept of Geostigma originated from Final Fantasy X, in a cut concept where Yuna traveled Spira healing diseases. The development staff included Geostigma as a plot device to motivate Cloud to fight to protect those weaker than him.
The Necrotic effect of the body's overcompensation to Geostigma bares similarities of a real life effect called a Cytokine Storm. The term Geostigma is reminiscent of a Stigmata (Ancient Greek for 'mark, spot, brand'), in Christianity. They are the appearance of bodily wounds, scars and pain in locations corresponding to the crucifixion wounds of Jesus Christ, such as the hands, wrists and feet.