An 82-year-old male nursing home resident with a history of mild Alzheimer's dementia is evaluated for a sudden change in mental status over the past 24 hours. Staff report he was at his baseline yesterday but is now alternately agitated and drowsy, does not recognize familiar staff, and is picking at his bedsheets and pointing at things that are not there. His attention fluctuates dramatically — he is unable to recite the days of the week backward and keeps losing track during conversation. Vital signs reveal a temperature of 101.2 degrees F. The most likely explanation for this acute change is: