Because of the large slave population's relative immunity and the acquired immunity of the white colonists (the first infection of yellow fever either "kills you or makes you immune") yellow fever ceased to be a major concern to the colonies after several generations. However, empires were forced to to send expeditionary forces into the Caribbean for warfare, because of the impossibility of keeping standing armies in the area. This provided yellow fever with the perfect hosts. Young adults, especially those from the northern hemisphere, were the most vulnerable population to yellow fever. .
Expeditionary forces were primarily made up of young adults, and their high concentration of previously unexposed Caucasians destroyed the herd immunity provided by large slave populations that tended to break the cycle of infection.7 Every expeditionary force that set foot in the American tropics was decimated by disease; some forces lost 75% of their men to yellow fever. Hence, yellow fever insured that no single empire could gain a monopoly over the region, but once the immune slave populations began to revolt the tides turned. Yellow fever had "constructed the geopolitical order form 1660 to 1780," but after 1780 it effectively "undermined" the power of empires because their foreign armies fell victim to the disease and were unable to defeat the immune revolutionary ex-slaves.
But how does any of this relate to the battle of Yorktown, which took place hundreds of miles north of the American tropics? On page twelve McNeill states that, "The vulnerability of expeditionary forces to tropical infection probably also helped create the United States." McNeill says that malaria, a mosquito transmitted disease, impacted the British expeditionary force in a similar fashion to yellow fever's effect on European forces in the American Tropics. This time the swampy rice plantations of the Carolinas provided the perfect breeding conditions for mosquitos; the British "prudently campaigned in the spring, aware of the enhanced disease toll of the hotter months, taking Savannah and Charleston by April.
