score:8
Probably chiefly because they didn't really know what was causing it.
The germ theory is actually fairly new, and even into the 20th century was quite controversial in non-scientific circles. It wasn't until public health officials working in the US Panama Canal construction zone managed to nearly eradicate the yellow fever that pretty much ruined previous efforts that public officials were convinced.
Prior to that, most people subscribed to the miasma theory. Under this concept, diseases were caused by "bad air", and best way to prevent disease was thought to be to keep things as clean as possible. Sometimes that meant using lots of water, which left lots of puddles for mosquitos to breed in...
Upvote:1
One reason is that leaders didn't know about particular diseases in relatively remote areas or how to fight/treat them.
In the examples you cited, European soldiers died of yellow fever in Haiti, and plague in Egypt. Those were tropical diseases that European commanders knew little about. In such cases, locals or "natives" had the advantage over them.
By the late 18th century, European commanders knew something about "nothern" diseases such as infections or scurvy, that could be controlled by sanitation in one instance, or citrus fruits in the other case. But Europeans would have problems in places like Latin America and Africa, where they did not know about, or at least fully understand, diseases peculiar to those climates.
Upvote:4
Most military commanders knew about diseases and did what they could do, or what they thought was right to do to prevent them. The large numbers of victims were due to insufficient knowledge available at that time. For example British admirals perfectly new about scurvy and about tropical diseases they encountered in the Caribbean and elsewhere. They were intensively looking for methods of prevention of scurvy, and tried various things. They tried everything, like sauerkraut etc., until they found that lemon juice helps and since the beginning of 19th century started to distribute it regularly to all hands. They knew that camping near a swamp is unhealthy and there is a rick of malaria. They did what they could to restrict or prevent the exposure of their people. But insufficient knowledge frequently led to large losses.
By the way the fact that the sailors rations included enormous amount of alcoholic beverages (by modern standards) is explained by the impossibility to preserve fresh water for long time. Roman army used vinegar instead.