This condition is caused by a virus called HTLV-1. It can make your brain and spinal cord swell, which causes the symptoms of the disease. Not everyone who has the virus will get HAM; in fact, some people also can carry HTLV-1 but have no symptoms.
People who get HAM usually live near the equator. You get it by coming into contact with blood or other body fluids of someone who has the disease. It’s not usually fatal, but you can die from it. Many people can live with the disease for decades.