I'm looking for accurate US county boundaries in shapefile (most likely), feature class, or a geodatabase. Specifically, I'm looking for Midwestern US states: Minnesota, Michigan, Illinois, Iowa, and Wisconsin.
You might find these cartographic boundary shapefiles for U.S. Counties useful: http://www.census.gov/geo/maps-data/data/cbf/cbf_counties.html
The Centers for Disease Control provide county-level shape files. https://wwwn.cdc.gov/epiinfo/html/shapefiles.htm They are in ESRI shapefile format but, CDC restricts their use to Epi Map.