The Digital Hazard isn't a virus. It's more like an error code that the overpopulation triggered. There are some Digimon that could cause the Hazard just by existing, too.

Yggdrasil's plan was to control how Digimon would grow in the new world, and how many and what kinds could live. But when it ran the X-Program, some...some Digimon mutated to become immune to it.