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Why did disease cause many deaths in the French Foreign Legion during the 1840s?

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French Foreign Legion soldiers in uniform with white hats and rifles stand in formation during a Bastille Day parade on the Champs Elysees in Paris. — French Foreign Legion Soldiers of the French Foreign Legion in a parade celebrating Bastille Day in Paris.

During the 1840s, the French Foreign Legion experienced high mortality rates primarily due to disease rather than combat. This was largely a result of the harsh conditions faced by legionnaires in Algeria, where they were frequently deployed to morale-sapping outposts and dispatched on expeditions to disease-ridden lands.1

At that time, the French army generally lacked adequate hygiene standards, and legionnaires were often utilized as labor troops in environments where they were exposed to infectious diseases unknown in temperate European climates. It was not until the latter part of the 19th century that medical science began to identify the transmission vectors for many of these tropical diseases, such as malaria and yellow fever, which were spread by insects like mosquitoes. Consequently, during the 1840s, the legion lacked the scientific understanding and medical infrastructure necessary to prevent or effectively treat the illnesses that decimated their ranks.12