Merlin's sickness

"All life signs have returned to near normal. Brain wave activity is nearing a conscious state. The problem is that her body appears to have absorbed... that is, the anti-entropic field appears to have contaminated her."

Sol rubbed his forehead. "Like radiation sickness?"

Dr. Singh hesitated. "Not precisely... ah... this case is quite unprecedented. Specialists in aging disease are due in this afternoon from Tau Ceti Center, Lusus, and Metaxas."

Sol met the woman's gaze. "Doctor, are you saying that Rachel contracted some aging disease on Hyperion?" He paused a second to search his memory. "Something like Methuselah syndrome or early Alzheimer's disease?"

"No," said Singh, "in fact your daughter's illness has no name. The medics here are calling it Merlin's sickness. You see... your daughter is aging at a normal rate... but as far as we can tell, she is aging backward."

Sarai pulled away from the group and stared at Singh as if the doctor were insane. "I want to see my daughter," she said, quietly but very firmly. "I want to see Rachel now."

–Hyperion (Dan Simmons)

Hyperion is largely a collection of interconnected character stories, and the Weintraubs' story (Sol's, Sarai's, and Rachel's) is one of my favorites. The idea of Merlin's sickness is neat, and the execution on this basic premise is excellent and ties in well with the rest of the book.