The Virus of Stone
The Lapillus Virus, better known as the Medusa Virus, is a form of Bloodstone spread through tiny flecks. Unlike actual bloodstone, this does not contain usable magic. Instead, it takes hold quickly and spreads throughout the victim's body, effectively turning them into living stone. They are still able to function normally, with arteries, veins, organs, etc. intact, they are just stone in flesh.
The virus spreads through flakes of cut stone - the bloodstone that's been cut away from someone and has been left without blood. It's light enough to be carried on a breeze, can stick to clothing and skin and even be breathed in. After it's attached, it can grow to encompass a lot of the bod in a matter of weeks.
This virus is thought to be the origin of Gargoyles. Or at least, the living ones.
Where it started
There is little evidence of how the virus began, but from tales passed down it can be inferred.
When bloodstone was first discovered, people didn't know the risks it would present. After the first bloodstone was cut away from someone and left without blood for weeks, it was assumed the stone would die. But they were wrong.
The stone disintergrated. The powder that was left behind was brushed and blown away, getting into peoples' lungs and on their skin. Very soon, many were overcome by the stone, and the town was shut down.
The cutaway stones are now dealt with effectively, but every so often the same thing happens and a small epidemic of Medusa Virus occurs. Many Bloodstone dealers and cutters are gargoyles for this reason.