Good question. Probably the barrier between the underworld and the surface world has loosened.

Making it easier for diseases carried by underworld youkai to spread to the surface.

But in any case, because humans burn their corpses, it was a huge problem.

Why?

Just like the name says, "Earth-dying Sickness" is an disease which "dies away" in "earth".

In other words, unless you bury the pathogens in the ground they will not die out.

If you try to cremate them, the wind will carry them everywhere...

It's a tricky disease like that.

Huh.

But apart from all that, what I'm really wondering about...

Yes?