The New York Times has published an interesting article on the case of 18 girls in Le Roy, New York (USA) who recently began manifesting a range of unusual symptoms, including tics, arm swings, and hums.  In some cases, the symptoms have been quite severe.  The case has attracted national attention in the United States, and a number of the affected girls have appeared on talk shows.  Experts have been divided on what the cause of the affliction might be, but many experts believe the case involves conversion disorder (formerly hysteria) and/or mass psychogenic illness.