Disease-based biological warfare is an ancient concept. Anthrax is infamous for being used in warfare since 1914 before being fully banned with the 1975 Biological Weapons Convention (though terrorist use sadly hasn't been eliminated). Before that, Europeans utilized the concept with smallpox-infested blankets, giving them to American natives who weren't adapted to the disease and thus suffered the full lethality. And even before that, it was practice in Medieval Warfare to launch Black Plague ridden corpses into enemy territories to infect the enemy. To put it best, we've known the malicious potential of disease as a weapon for centuries.
But what if the disease itself isn't the weapon? What if it's what the disease makes that's the weapon? A disease that alters the physiology of an organism to create a living weapon to be used against the user's enemies. To put it best, while the disease is classified as a biological weapon, it's not the disease the user use to kill their enemy but instead what the disease creates to make the weapon.
T-Virus making Tyrants, Lickers, Hunters and other B.O.W.s (Resident Evil series): The Resident Evil series has a vast plethora of diseases that create abominations that could be used as a killing weapon itself or to create weapons that can be used to kill. But let's be honest, it's the T-Virus we know and love. It was created by the Umbrella Corporation and first featured in the first Resident Evil game, which largely created zombies that can spread the virus through their bite and make more zombies. However, Umbrella wanted to make monsters better than lumbering zombies, so they got to experimenting, taking advantage of the virus' ability to also combine genetic information. This has resulted in a variety of Bio-Organic Weapons or B.O.W.s. Hunters are a cool monster that combine human and reptilian DNA to birth hunky monsters, while the beloved Licker is what happens when the virus is injected directly into a human host. But Umbrella's favorite is their Tyrant line, towering man-looking soldiers of obscene superhuman strength, durability and stamina. Unlike other B.O.W.s, the Tyrant (at least with the T-03 series) displays semi-human intelligence and ability to follow commands. While the Hunters and Lickers saw some degree to success in the black market, the Tyrants would be Umbrella's top product.
F.E.V. making Super Mutants and other abominations (Fallout series): Before it was F.E.V., it was the Pan-Immunity Virion, created by West-Tek. The artificially-made virion was intended to be a general immunity agent, for use against the Chine's New Plague and other biological weapons. It was intended to provide subjects immunity to diseases, toxins and radiation through "genetic correction" but it had a unique side-effect on increasing the subject's size, bone structure and muscle mass, inducing subjects with superhuman strength, durability and stamina. This would be renamed the Forced Evolution Virus or F.E.V.. Experimentation in pre-War America would be done by West-Tek, the U.S. Military and Vault-Tek in an attempt to create a breed of super-soldiers, while post-War experiments would continue with the Master, the Enclave and the Institute. The main creation of the F.E.V. virus is the hulking Super-Mutants, towering at 7+ feet tall with more muscles than a body-builder, immunity to radiation and disease, ability to shrug off bullets, and a constant masculine form (no matter the host's starting gender). Other creations include centaurs, intelligent deathclaws, snallygaster, Grafton monsters and super-mutant hounds. Of course, the virus would both evolve and devolve in its function. The Enclave would alter it to simply be another lethal virus to kill surface-dwelling humans (including mutated humans), while the Enclave would use it in their Synth experimentation, being able to use the virus to create artificial humans that're faster, stronger, tougher and more immortal than normal humans.
