Pandemics would be pretty unlikely in the days of such low population density. Epidemics definitely happened pretty routinely though.

There was a bubonic plague pandemic (1346-1353) right after the time this book was wrtten (1339-1345 according to OP).

Sure, but the topic was pandemics we don't know about because they weren't written down. The bubonic plague was written about pretty extensively. Even earlier disease outbreaks were written about.

I think the topic was how pandemics would cause word of mouth stories to be lost, not the story of pandemics.

It could be worse, it could be pestilence of biblical proportions.

Oh wait . . .