The Crusades brought diseases back from the Middle East which decimated the European population and social structure.
Nuts!
No, it was sailors from the Black Sea who brought the Black Death.
The Crusades threw the moslem hoardes out of many of the Christian countries that they had conquered. The only problem with the crusaders is that they did not have nuclear weapons.
Of course there's plenty of controversy about the exact timeline and causes, but from what I've read it took root in the Crimea in 1347. (Not an area touched by the Crusades.) It was spread by the Mongol army (Chinese, not Arab: also not involved in the Crusades) when they catapulted the infected corpses over the city walls of Caffa (Crimea) to infect the inhabitants. Genoese traders /commercial travelers fled by sea, bringing the plague by ship into Sicily and the south of Europe. It spread north from Mediterranean port cities, via trading routes, reaching Oslo by 1350.
Spread by commercial travelers and sailors. Not Crusaders.