An 82-year-old woman was admitted to hospital 3 days ago for treatment of a urinary tract infection (UTI). She has a background of mild cognitive impairment and hypertension. The nursing staff report she has become acutely confused overnight. She is agitated, attempting to climb out of bed, and describes seeing insects on the walls. Her level of consciousness appears to fluctuate. Her vital signs are stable apart from a temperature of 37.8°C. What is the most likely diagnosis?