A morbillivirus has been isolated and added to an increasing list of e
merging viral diseases. This virus caused an outbreak of fatal respira
tory disease in horses and humans. Genetic analyses show it to be only
distantly related to the classic morbilliviruses rinderpest, measles,
and canine distemper. When seen by electron microscopy, viruses had 1
0- and 18-nanometer surface projections that gave them a ''double-frin
ged'' appearance. The virus induced syncytia that developed in the end
othelium of blood vessels, particularly the lungs.