It can be difficult to distinguish between delirium, dementia, and depression. Dementia and depression are usually slower onset, chronic illnesses, while delirium is an acute state with relatively rapid onset. Classically, delirium is caused by an infection or drug toxicity. In this case, the patient has known Alzheimer's disease, which is a form of dementia. On top of that, the person also presumably has pneumonia which is causing a transient delirium. Cognitive functioning should return to baseline after the infection is cleared and the delirium has passed; however, that baseline will be different then someone without dementia. Even though the patient has refused to eat, this does not necessarily indicate depression and could be a symptom of delirium due to infection.