The fourth patient in the world has completely recovered from HIV infection.

This is a 66-year-old man who became ill back in the 1980s. He was helped by a bone marrow transplant - it was done to treat leukemia. The donor had a rare DNA mutation that makes the body immune to the virus. He is called the "patient from the city of hope" - in honor of the Californian Duarte, where the man was treated.