Nat Torkington says,
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.
Incidentally, if you've ever wondered what the "ague" is that Shakespeare kept banging on about–it's what the English called malaria. It was widespread in the marshlands during Shakespeare's time. (via sciblogs)