My character, J'hal Fergot, (Account: Wicked Witch of the West) Cast Contagion on a cave beetle and the DC of it was fifteen.
My PC has GSF necromancy and had 20 wisdom at the time. So the DC should have been at least 22.
She was a druid, who cast it at a third circle.
Contagion randomly selects one of the custom diseases around. The DC is based upon the random disease and not the spell itself, thus the DC being low or random.
However, it would be nice if the focus feats could boost them, or even if there was a way to select which disease is cast.
Being able to select deadly diseases with GSF necromancy would be pretty pimp.
Remembering correctly, the custom EFU diseases are extremely powerful, have a proximity DC to an infected person to represent contagiousness, and have special effects that a lesser restore won't remove.