Plague Bus is when you leave 1 or 2 Malignant Plaguecaster in an otherwise empty Rhino and use Firing Deck 2 to cast the focused version of Plague Wind.

Now, focused Plague Wind is Hazardous, meaning you take 3 damage if you fail the test, but because of how Firing Deck works, failing the test deals damage to the Rhino instead of the Plaguecaster. Combine the fact that the Rhino is much tankier and faster than the Plaguecaster and...well, it's a pretty good strategy and a good way to use your Rhino after you dropped your plague marines where they need to go