Viral Syndrome: Given the acute onset of fatigue, body aches, fever, and the presence of left lower extremity weakness, a viral syndrome such as a flu-like illness or another viral infection affecting the nervous system (e.g., West Nile virus) could be the most straightforward explanation. The symptoms are non-specific and could align with a variety of viral infections.

Multiple Sclerosis (MS): Although MS can present at any age, its peak onset is in young adulthood. The left lower extremity weakness could be a manifestation of a relapse in someone with previously undiagnosed MS.
Lumbar Disc Herniation: Acute back pain and lower extremity weakness can be symptoms of a lumbar disc herniation, which could be causing nerve root compression leading to the weakness.
Infectious Meningitis: While the presentation might not be classic (e.g., headache, stiff neck), atypical presentations can occur, especially in adults, and the fever and body aches could be indicative of an infectious process affecting the meninges.

Stroke: Although less common in a 48-year-old without significant past medical history, a stroke (ischemic or hemorrhagic) could present with sudden weakness, especially if it involves the motor cortex or corticospinal tract. The absence of typical risk factors does not rule out stroke.
Spinal Cord Infarction: Similar to stroke, spinal cord infarction is a vascular event that could cause acute weakness and must be considered due to its potential for severe and permanent disability.
Guillain-Barré Syndrome: An autoimmune disorder that can follow a viral infection, leading to ascending paralysis. Early recognition is crucial for appropriate treatment.
Cauda Equina Syndrome: A serious condition that requires urgent surgical intervention, characterized by low back pain, sciatica, and signs of nerve root compression (e.g., weakness, bladder/bowel dysfunction).

Neurosyphilis: In the context of left lower extremity weakness and systemic symptoms like fever, neurosyphilis (especially tertiary syphilis) could be considered, although it is less common.
Lyme Disease: If the patient has been exposed to ticks, Lyme disease could be a rare cause of her symptoms, including the weakness, through its effects on the nervous system (e.g., Lyme meningitis, radiculopathy).
Vasculitis (e.g., Giant Cell Arteritis, Polyarteritis Nodosa): These conditions can cause a wide range of systemic symptoms including fever, body aches, and could potentially lead to neurological manifestations due to vascular involvement.