But as a supervillain, it's far more evil to subjugate a person to a lifetime of torture and ruined existence than to outright kill them. Plus, once cancer cells and all start multiplying, the disease villain can just sneeze himself out and infect his next victim. Or if you're a parasitic worm, just lay a hundred eggs, poop yourself out of your host, and go infect someone else.
Yeah, being able to transmutate into a disease or a parasite is one frightening superpower. Any superhero would have a rough time against a villain who can do that.
Despite all the cool powers people have spoken of, I still stick with wanting the power to be in several different places at once.