Yellow fever epidemics struck the United States repeatedly in the 18th and 19th centuries. The disease was not indigenous; epidemics were imported by ship from the Caribbean. Prior to 1822, yellow fever attacked cities as far north as Boston, but after 1822 it was restricted to the south. Port cities were the primary targets, but the disease occasionally spread up the Mississippi River system in the 1800s. New Orleans, Mobile, Savannah, and Charleston were major targets; Memphis suffered terribly in 1878. Yellow fever epidemics caused terror, economic disruption, and some 100,000-150,000 deaths. Recent white immigrants to southern port cities were the most vulnerable; local whites and blacks enjoyed considerable resistance.