"Over the next two centuries, hundreds of thousands of Western Europeans marched and sailed to the eastern Mediterranean - to a region with peoples, cultures, biota, and diseases quite different from those to which most of them were accustomed..."
"The Crusades...were also the first massive attempts to permanently extend European power outside the boundaries of that continent, and they produced four new states in the biblical lands...Today, the only evidences left of these states are a few massive ruins, usually the stumps of castles. Western Europe's first imperialistic attempts in Asia failed, and failed because of factors similar to those that rendered later European extensions in Asia ephemeral."
"In September and October of 1098, thousands of members of the First Crusade died in some sort of pestilence. It seems to have been infectious: An army of 1,500 Germans, freshly arrived, suffered rapid annihilation, which suggests infection rather than malnutrition, although the latter certainly could have contributed to the rapid rate at which they died....The chief killer during the Seventh Crusade, on the other hand, probably was malnutrition. The symptoms - sore mouth, rotting gums, stinking breath, skin "tanned as black as the ground, or like an old boot that has lain behind a coffer" - suggest a diagnosis of scurvy. But such diagnostic hindsight is largely guesswork. The Crusaders' descriptions of their sicknesses are ambiguous, and no doubt many pathogens were operating at once. The Franks, when they traveled to the East, were subject to a new climate, exposure to all sorts of severe weather, a new diet, malnutrition and occasionally starvation, exhaustion, general disorientation - stress in a host of forms - in addition to new pathogens."
"In contrast to the Franks, the Saracens fought on their own ground. Richard of Devizes noted enviously that "the weather was natural to them; the place was their native country; the labour, health; their frugality, medicine."
When the Crusaders arrived in the Levant, they had to undergo what British settlers in the North American colonies later would call "seasoning"; they had to ingest and build resistance to the local bacterial flora. They had to survive the infections, work out
modi vivendi
with the Eastern microlife and parasites. Then they could fight the Saracens. This period of seasoning stole time, strength, and efficiency and ended in death for tens of thousands.
It is likely that the disease that affected the Crusaders the most was malaria, endemic in the Levant's low, wet regions and along the coast, exactly where the bulk of the population of the Crusader states tended to concentrate. Crusaders from the Mediterranean and even from northern Europe may have brought with them a degree of resistance to malaria, because the disease was widely distributed in medieval Europe - indeed, it was still present as far north as the English fen country in the nineteenth century - but nowhere north of Italy, certainly, was it as virulent as continually and in so many varieties as in the eastern Mediterranean. Unfortunately for the Crusaders, a person immune to one kind of malaria is not immune to all, and immunity malaria is not long-lasting."
"Longevity was not characteristic of Crusaders. Frankish women apparently did better than Frankish men in the East, but they often failed to produce healthy children, or any children at all. It is pertinent to mention that malaria is a great threat to pregnant women, often causing abortions, and it is very dangerous for children. The inability of the women to produce guarantees of the future made all efforts in the present irrelevant. The Crusader states died like bowls of cut flowers."
"Frankish imperialism had its decades of triumph, and the Crusader presence in the Holy Land lasted as long as or longer than European overlords held sway in Algeria and India in our era. But the Crusader states ultimately failed. Not even Latin Christian fanaticism could cancel out the numerical advantage of the indigenous peoples. Europeans might be able to temporarily conquer, but never permanently depose, the more numerous native populations, especially with the disease environment working against the invaders.