Sickle cell anemia is caused by a genetic mutation. When a person carries only one mutated copy of the gene, they don't have any symptoms but have a 50% chance of passing the abnormal gene to their children. If two carriers have a child, that child has a chance of getting both mutated copies, one from each parent, which causes the disease.
Every year, around 300,000 babies are born with sickle cell anemia, which causes red blood cells to be deformed. The diseased cells have a crescent shape, causing them to get stuck in blood vessels and die off prematurely. The resulting impairment in blood flow can ultimately cause other problems in patients, including organ failure and stroke.
Recent work has indicated that all cases of the disease trace back to a single individual; that person was probably born in West Africa about 7,300 years ago.