It was an extraordinary experience, for it meant that in an instant I was not only connected to her physically but in every other way as well. And I saw that her body was wonderous, with each part working perfectly with the others as she moved sinuously beneath me -- except for one. For there, lurking deep in the central part of her brain, was a dark shadow.

I zoned in on it to find a cluster of astrocytes, deep and low in the left parietal lobe They had mutated and, even as I watched, pulsed and vibrated like a hornet's nest as they multiplied. Can you imagine my anguish,...