Why did more U.S. soldiers die from disease than from combat during the Mexican-American War?

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During the Mexican-American War, U.S. soldiers suffered significantly higher mortality rates from disease than from combat, with at least 10,000 deaths attributed to illness compared to approximately 1,500 from battle-related causes.1

Several factors contributed to this disparity:

Poor Sanitation: Inadequate sanitary practices were a primary driver of disease transmission. Volunteers, who often lacked the discipline of regular army troops regarding hygiene, experienced higher mortality rates.1
Virulent Diseases: Troops faced outbreaks of various illnesses, most notably yellow fever, which was particularly deadly. Other common diseases included measles, mumps, and smallpox.1
Lack of Immunity: Many soldiers originated from rural environments and lacked the developed immunities to these diseases that their urban counterparts possessed, making them more susceptible to infection when exposed in new environments.1

The high toll of disease highlights the severe logistical and health challenges faced by military forces during 19th-century conflicts, where environmental factors often proved more lethal than enemy fire.1