When scientists handle variola, international rules require them to wear full space suits and to be inside a sealed Biosafety Level 4 containment zone. The WHO forbids any laboratory from possessing more than ten percent of the DNA of variola, and no one is officially allowed to do experiments with smallpox DNA. Variola is now exotic to the human species, highly infective in humans, lethal, and difficult or impossible to cure. It is generally believed to be the most dangerous virus to the human species.
