Hi Alex M.,
"Susceptibility" is a suspect term here, I think. The individual devils are each no more nor less capable of succumbing to any particular disorder, be it viral, bacteria, fungal, or nasty cancer-transmitted-directly-by-biting, than before the DFTD onset. There's no herd immunity here and the mortality once bitten is high. However, the RATE of SPREAD or PROBABILITY of TRANSMISSION of any condition is diminished by the lower population density, assuming of course that the devils have no fatalistic urge to seek out other devils to be bitten by. They simply won't be encountering each other as often.
Further than that, I guess you could say that the Details are in the Devils?
-- Cheers, -- Mr. d.