[Ergotism][1] fills all your requirements.

Fire people eat black rye bread. I'd expect fire land to be a dry place and fire people not to know about ergot. But in the moister earth soils, fire cultivation methods will make the ergot thrive.

When they started to seize the earth plains, the fire guys started farming them with cheap labor (exactly as Romans did when they conquered Sicily) and send the crop to feed their army and cities. They won’t use crop rotation and they won’t double-check their crops for fungi. The downside is that it needs at least one year after the invasion for the epidemic to start, maybe more. 

Basically, the harvest is just after the foreboding unexpectedly strong rain season. Good point is, it won’t be traced back, as the flour will be sent to be sold in the capital and mixed with non-contaminated one before getting sent back to the soldiers. And who would expect honest fire peasants cultivating rye, as they have always done, to be the culprits? (In the real world, it took them centuries to find it out.)

Another good point is, the more you’re involved with the invasion (being a soldier, buying goods from invaded lands), the greater the chance that you’ll be eating contaminated bread, without this being a specific rule.

Earth people are pretty much immune to ergotism, they eat white wheat bread, the whiter the better. Just make Bob stumble upon a sack of contaminated wheat, discarded because it's not white enough.

  [1]: https://en.wikipedia.org/wiki/Ergotism