Botulism
Botulism is a killer and is caused by one of the most common bacterium known to science, Clostridium Botulinum a natural contaminant of most wild bird cadavers.
When this bacterium comes into contact with any decaying flesh (ie meat), it causes a deadly toxin to be formed. If this flesh is then eaten by an animal, the toxin affects its victim by attacking the animals nervous system causing paralysis, at first usually in the hind legs, eventually affecting the body's vital organs and leads inevitably to the death of the affected animal.
There is no cure or treatment for this and ferrets are amongst the most susceptible animals to botulism.