Here is a Library of Congress map showing prevalence of Malaria throughout the United States in the 1870s. Astonishing how dangerous it was. In some places, it accounted for one-seventh of all deaths. People thought it was bad air (literally, "mal-" + "air"), figured out it was a parasite in the blood in the 1870s, but it wasn't until 1908 that a Cuban doctor made the connection with mosquitoes.