Dreaded Diseases.

Yellow fever and malaria, two of the most feared diseases of the period, were often confused. Their symptoms—chills, aches, and high fever— are similar, and they tended to appear in hot weather. Neither is contagious—one person cannot catch it directly from another person—and both are transmitted by mosquitoes. In the late eighteenth century no one knew what caused these diseases; some thought there was a connection with swamp air or with summer heat. Yellow fever attacks the liver, producing the jaundice that makes the skin turn yellow, while with malaria, parasites build up rapidly in the liver, and the overworked spleen becomes enlarged as it tries to filter the parasites out. In severe cases blood vessels to the brain are blocked, causing delirium, coma, and death. The two diseases were apt to strike differently: yellow fever broke out in frightening...