HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a progressive disease of the nervous system that affects less than 2% of people with HTLV-1 infection. Signs and symptoms vary but may include slowly progressive weakness and spasticity of one or both legs, exaggerated reflexes, muscle contractions in the ankle, and lower back pain. Other features may include urinary incontinence and minor sensory changes, especially burning or prickling sensations and loss of vibration sense. The reason some people with HTLV-1 infection develop HAM/TSTP is not well understood.