"Avian Encephalomyelitis is a relatively rare infectious disease of chicks caused by a picornavirus. It's chief symptoms are loss of coordination and rapid trembling of the head and neck, giving the disease its common name, epidemic tremor. The disease is transmitted from breeders to chicks through hatching eggs. An adult outbreak is so mild it is likely to go unnoticed, lasts less than a month, and leaves birds immune without making them carriers. Survivors of a chick outbreak should be culled, since they rarely develop into good layers or breeders." -The Chicken Health Handbook by Gail Damerov