Within the reams of important and interesting data collected by the United States Census who lives where; what (race, gender, age, etc.) makes up who is at least one interesting metric: median household income by county.
As a handy chart over at The Atlantic points out, "Six of the ten richest counties in America are in Virginia and Maryland, clustered around the Washington, D.C., metro area" (according to 2012 estimates).
Rich, here, doesn't necessarily mean richest the counties around the District are home to a uniformly large concentration of high-earners but not the "vertiginously rich" found in California or New York, whose averages are deflated by low- or middle-income households.
Of the poorest counties, six are in Mississippi and Kentucky.
A seventh is in Alabama: Sumter County (including Cuba, Emelle, Gainesville, Geiger, Livingston and York; and home to the University of West Alabama), with a population of 13,427, has a median household income of $24,059. Some context: the poverty line for a four-person household in 2013 is 23,550.
Sumter is ninth on the list of 10 poorest counties, behind only Mississippi's Quitman County.