The problem of housing affordability, long a concern in popular big cities, has moved to rural America. Nearly one-fourth of the nation’s most rural counties have seen a sizeable increase this decade in the number of households spending at least half their income on housing, a category the federal government calls “severely cost-burdened.”Those counties, none with towns of more than 10,000 residents, have experienced housing cost increases significant enough to force families to scrimp on other necessities.Meanwhile, only two big-city counties — Bronx, New York, and Norfolk, Virginia — fell into the same category. Both had 2-point increases, according to a Stateline analysis of American Community Survey estimates from the U.S. Census. Stateline compared the early years of the Great Recession, 2006-2010, with the most recent economic recovery era, 2013-2017.