The nutritional disorder vitamin B12 deficiency and the infectious disorder human immunodeficiency virus may both cause similar mixed syndromes that develop over months to years. These disorders may cause polyneuropathy, dementia from diffuse loss of cerebral cortical neurons, or degeneration of certain parts of the spinal cord. The myelopathy that may occur with these disorders is unusual enough to get its own name of combined degeneration of the spinal cord, which may lead to somatosensory and upper motor neuron abnormalities. This involves the fairly selective loss of upper motor neuron axons, and a subset of somatosensory axons that carry information about touch, position, and vibration sense. Because upper motor neurons axons may be affected in the spinal cord, and lower motor neuron axons may be affected in the nerves, these disorders may have a mix of upper and lower motor neuron abnormalities on examination. The immune system dysfunction that may occur with the human immunodeficiency virus may also lead to certain types of infections or neoplasms involving the central nervous system, so that many other neurological syndromes may also occur.