A new wave of therapies can exert a magnetic hold over disease — literally. The therapies employ powerful, roughly spherical magnets to help kill carefully targeted diseased cells and nothing else. What makes these magnets special is their size. Each is about a thousandth the diameter of a human hair.

Most researchers in the field are designing these billionth-of-a-meter-scale magnets to serve as highly localized space heaters. Under the influence of an external magnetic field, the