which ones were those? nobody in Europe was initially immune to the bubonic plague pandemic in the 14th century (lasting until the 17th), just like they weren’t immune to the first plague pandemic in the 6th century, both of which devastated islamic and Christian lands. all of Eurasia is basically the same disease pool. (the crusades took place between those two pandemics) “exposed” means “causes devastating epidemic and death of 30-50% of the population from the initial exposure”, so if you think this is a benefit, then nice going, crusaders. don’t do anymore favors.