The Marvel Universe is full of dangerous forces and powerful figures. Even the more mundane and simple of power-sets can be turned into deadly weapons when given the right opportunity. X-Men in particular can become absurdly dangerous when utilized in the right way.
Two variants of classic mutant heroes became the worst nightmare for their entire reality after they were infected with a Techno-Organic virus -- setting up the Vi-Lock Virus as one of the Marvel multiverse's most potent threats.
Earth-8545 was introduced in Exiles #20 by Judd Winick and Jim Calafiore. In this reality, the Legacy Virus was also released and proved far more deadly than in the core-Marvel Universe. Countless mutants suffered and died from the disease. Desperate to save an infected Doug Ramsey, the alien Warlock fused with his best friend. But this only further mutated the virus, which became known as the Vi-Lock Virus. Now capable of spreading to humans and aliens as well, the Vi-Lock Virus was a thinking disease. As it spread further across the world and infected more people, the victims were transformed. Now part of a techno-organic hive mind known as the Network, each Vi-Lock would be driven to spread the disease even further.
Notably, the infected retained their intellect and powers but were driven by the disease to further spread chaos. In essence, they operated similarly to the Marvel Zombies, albeit with more unity in their purpose. Within five years, almost every notable X-Man, Avenger, Inhuman, and member of the Fantastic Four -- as well as all their villains -- had been exposed to the Vi-Lock Virus and transformed into an agent of the Network. In estimation, roughly three-quarters of Earth's population was lost to the disease. Making matters worse, one of the most dangerous mutants to be exposed to the disease, Forge, was among the infected. Due to his mutant ability giving him an innate understanding of all technology, Forge quickly became recognized as the leader of the Vi-Lock.
Soon, the disease spread across the entire world, with only a handful of holdout heroes remaining underground as a resistance movement. Although an antidote was eventually developed by a surviving Hank Pym, it wasn't a pure cure. It wouldn't be until the arrival of the reality-hopping Exiles that any hope would be found, as their knowledge of other worlds gave Morph the idea to summon the Asgardians for aid in reversing the virus. The Vi-Lock Virus is among the most terrifying plagues to ever be introduced in the Marvel multiverse. On top of corrupting its victims and further spreading the virus, its continued exposure across the world made it a dangerous enough means of breaking humanity and mutants alike.
The saddest aspect of the disease might just be the unlikely figures who made it worse. Cypher and Forge both lack offensive mutations, instead typically using their powers to spread understanding or improve the lives of mutants respectively. In theory, neither of them should be considered highly dangerous. Instead, because of their exposure to a disease they couldn't possibly control, the world fell into chaos. Even when the reality-hopping Nocturne and Sunfire briefly revisited the reality months later, the remnants of the Vi-Locks remained -- a band of monstrous infected, still trying to consume what life-force they could find. The Vi-Locks are among the Marvel Multivere's most dangerous plagues, even more so than the Hunger Gospel from the various Marvel Zombies storylines.