On this 6.1.0 playthrough, every time I rest anywhere in the Moathouse, including the green tent area in the tower, Ronald catches a disease. Only Ronald, and every time he rests. If he rests twice, two new diseases. Nobody else is ever affected. Is this a new twist? Maybe the unholy aura of the place tend to infect good clerics more often? I guess that would kind of make sense. Kind of.
No, his saves are decent. I'm willing to believe it's sheer random chance. I've stranger random results than that.
All diseases are caught somewhere, but it takes days before they develop. If fact, you can have several diseases that run parallel to each other and end at different times. Rats, stirges, ghouls, and giant ticks all cause diseases. Resting several days will get rid of them.
All that said, the game does at times have an odd (and of course inconsistent) tendency to be bull-headed about seemingly random events. There have been times when I've been trying to test something and traveling on the world map and the game does the "by god, we will have a random encounter, right here, right now" thing every time I reload, in the same spot on the map and with the same monsters. I assume that means the RE was actually queued before the save I keep reloading, so maybe that's the case with Ron too. Interesting to think that the game might actually consider REs before you open the world map though.