

Lastly, the powers may just inherently damage the body when used, either as a mystical price or because they are not designed for human use (or rather, the body wasn't given the Required Secondary Powers to handle using the primary ones).

Maybe it was thanks to Psycho Serum, or the machine/amulet that gave them their powers was broken. Other times it's related to having somehow gained superpowers. It may be that the process used to make the Clone, Half-Human Hybrid or genetically enhanced character with Super Soldier powers had an inadvertent (or intentional) design flaw (or "birth defect") that slowly reduces their lifespan, limiting their lives to years or months. This is what happens to characters with Power Degeneration. Now imagine that cancer can grow faster whenever you do what you're best at: dancing, painting, coding software, kayaking. Imagine cancer, that slow insidious death that kills you from the inside, but which is also intrinsically you it is a body eating itself, and unable to heal.
