The plague spreads through the mind rather than by germs, meaning that infection spreads at a very fast rate. The plague appears at first as a zombie plague with survivors. These "survivors" may appear to be immune, with any
Imperial Guard protectors quickly getting infected and the survivors being torn apart rather than infected. The zombies will also ignore
Space Marines; the plague exploits a grain of its victim's doubt in the
Emperor - hence it knows better than to look for this doubt in the
Adeptus Astartes, who carry a fraction of the Emperor Himself in their
geneseed.
[1a] The Doubtworm's victims later turn into humans again, appearing to resist the other zombies, and even grow in numbers.
[1b] In the second stage of infection, they mutate into a worm, noted to have many eye-mouths and one giant eye.
[1c]