AIDS is responsible for grotesque cancers and infections. Polio paralyzed young children. Second, at the moment of the panic before much is learned about the disease's origin everyone seems vulnerable, and it's not clear that prevention is possible. And finally, the disease organism is new and weird and seems to have sprung from a dark, mysterious place. AIDS is a creepy mutating monkey virus.
Mad cow is similarly vicious, unstoppable, and mysterious.
It murders by driving its young victims insane, then melting their brains. It theoretically puts anyone who ever ate English beef at risk. It was spawned in the miasma of rendering plants and slaughterhouses, our own hell's kitchens. And the disease organism is a mystery. Whatever it is, nobody has the vaguest idea how to kill it.