From Dulles International Airport:

Fort Detrick is located in the Northwest section of the city of Frederick. From Dulles International Airport: Exit I495 to I270 North, to 15 North to Frederick and exit 7th Street and make right and keep straight to the Fort Detrick gate.

From BWI:

From Baltimore Washington International take I695 North to I70 West to Frederick and take 15 North to 7th Street.