Medical investigators have good reason for suspecting that viruses may cause many common and baffling disorders of the human nervous system, to say nothing of some forms of cancer. But indicting the culprits has proved to be incredibly difficult. Most of the diseases—such as multiple sclerosis, the amyotrophic lateral sclerosis that killed Lou Gehrig, parkinsonism, and perhaps myasthenia gravis—do not normally attack animals, so it is next to impossible to study them in the laboratory.
Now, with patience and prodigious efforts extending halfway around the world, researchers at the National Institute of Neurological Diseases and Blindness have managed to inject lab...