A 64-year-old man with a travel history to Southern Minnesota is brought to the hospital's emergency room on July 15 by his brother. The brother said that the man had a 2-day history of fever, headache, and vomiting, but today he appeared confused. He is confused by some of the simple questions you ask him. His spinal tap is clear with 75% PMNs and a head CT is normal. The most likely cause of his symptoms is: