Peripheral Neuropathy: This is often the most common cause of numbness and weakness in a specific limb, such as the left calf. It can result from various factors including diabetes, vitamin deficiencies, or trauma to the nerve.

Lumbar Radiculopathy: Compression or irritation of the nerve roots in the lumbar spine can cause symptoms in the calf, depending on which nerve root is affected. This is a common condition that can result from disc herniation, spinal stenosis, or spondylolisthesis.
Peripheral Artery Disease (PAD): Reduced blood flow to the legs can cause pain, weakness, and numbness, especially during walking. PAD is more common in smokers and those with diabetes or high cholesterol.
Muscle Strain or Tear: Direct injury to the muscles in the calf can cause weakness and numbness if the injury affects the nerve supply to the area.

Cauda Equina Syndrome: A serious condition where the nerves in the spinal canal are compressed, leading to loss of function in the lower limbs. It requires urgent medical attention to prevent permanent damage.
Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the leg can cause swelling, pain, and numbness. It's a medical emergency because it can lead to pulmonary embolism if the clot breaks loose.
Spinal Cord Infarction: A rare but serious condition where the blood supply to the spinal cord is interrupted, leading to loss of function below the level of the infarct.

Neurosyphilis: A complication of untreated syphilis that can affect the nervous system, causing a variety of symptoms including numbness and weakness.
Vasculitis: Inflammation of the blood vessels can affect blood flow to the nerves and muscles, leading to numbness and weakness.
Tumors: Both benign and malignant tumors can compress nerves, leading to numbness and weakness. This would be a rare cause but should be considered in the differential diagnosis, especially if other symptoms are present.