This is a disease produced by infection and leads to the quick and inevitable death of the animal.
You can recognize that the animal suffers from diphtheria when it remains with its beak open ad its breathing shows signs of great agitation. In addition, it keeps its eyes closed, sneezes frequently, shakes and diarrhea.
As soon as these symptoms show, you must remove the animal from the cage quickly in order to avoid that it infects the other specimens. Always check with veterinary for diagnosis, and the most appropriate way to disinfect the cage to prevent new cases.