Not that Penric knew what a genestealer was, being centuries too early from modern biology -- he'd grasp the idea of offspring inheriting traits from their parents, but not the mechanics. But 'stealer' could suggest why soldiers were needed, if it was a literal theft, possibly with violence. And the association with disease in Gadriel's mind suggested it had something to do with health or the body.

He nodded, showing he was listening. "Can I ask where the power saints use to answer prayers come from? Since your Emperor has those like you."

(Penric was trying not to make assumptions, but the loose tie was to the old spirit warriors and shamans of the Weald. Not so much in the mechanics, but as the only example he could think of where someone became something slightly removed from normal people (mostly) voluntarily -- saints required a certain set of circumstances, and while the Temple tried to plan sorcerers, it didn't always work.)