Urinary Tract Infection (UTI): Given the age of the patient, UTIs are common and can present with non-specific symptoms such as fever, confusion, and lethargy, especially in the elderly where typical symptoms like dysuria may be absent.

Pneumonia: A common infection in the elderly that can cause fever, confusion (due to hypoxia or the infection itself), and lethargy. The presentation can be atypical, without the classic symptoms of cough or chest pain.
Sepsis: Although it can be caused by any infection, in the context of an elderly patient with fever, confusion, and lethargy, considering sepsis is crucial. It may not always present with the typical signs of infection or organ dysfunction early on.
Dehydration: Often seen in elderly patients, dehydration can lead to confusion and lethargy. While not an infection, it's a critical condition to identify and treat promptly.

Meningitis: Although less common, meningitis is a medical emergency that requires prompt diagnosis and treatment. The presentation can be subtle in the elderly, with fewer typical signs like neck stiffness or headache.
Encephalitis: Similar to meningitis, encephalitis is a serious condition that can present with non-specific symptoms such as fever, confusion, and lethargy. It's crucial to consider, especially if there's a history of exposure to certain pathogens.
Intracranial Hemorrhage or Stroke: While not typically associated with fever, these conditions can cause sudden onset of confusion and lethargy. Fever might be a secondary response to the brain injury.
Medication Side Effects or Overdose: Certain medications or their interactions can lead to confusion, lethargy, and even fever. Given the polypharmacy often seen in elderly patients, this is a critical consideration.

Subdural Hematoma: More common in the elderly due to falls, it can present with non-specific symptoms including confusion and lethargy. Fever might not be a primary symptom but could occur due to associated infections.
Neurosyphilis: A rare condition but one that can present with a wide range of neurological symptoms, including confusion and lethargy. It's more likely in patients with a history of syphilis.
Creutzfeldt-Jakob Disease: A rare, degenerative, fatal brain disorder that can present with rapid decline in cognitive function, including confusion and lethargy. Fever is not typically a primary symptom but can occur.