a serious viral disease of birds caused by a paramyxovirus and marked by respiratory and nervous system problems. Some adult birds recover, although mortality rates are high in tropical and subtropical regions. Young chickens are especially susceptible and rarely survive. Signs are variable in turkeys and almost absent in ducks. There is no effective treatment. Vaccines are available and are given repeatedly for best protection. Humans can become infected by handling sick birds but usually develop only a temporary conjunctivitis (inflammation of the mucous membrane lining the inner surface of the eyelid).