where did he store the cats?
They were probably ships cats, kept on board to keep the rats from destroying the provisions, ropes, etc.
Plague traveled by land. Or at least the only one I know of definitively tied to rats did (Black Death/Bubonic Plague). It started somewhere around in Asia and spread West Via land-trade routes.
Plague traveled by both. It came from east Asia and was first recorded in Europe during a Mongol siege of a Genoese trade post in Crimea. From there it spread from ships to the great merchant cities of Genoa, Venice, and Naples after which it spread by both land and sea along the trade routes.