People of certain ethnic origins are more likely to get beta thalassaemia. It occurs more often in areas where malaria is common, eg Middle East, India, South East Asia(incl southern China, Thailand and Malaysia), parts of Africa and some Mediterranean countries. Scientists think changes happened in the genes of people in these areas that helped to stop the parasite that causes malaria from multiplying and attacking red blood cells. It's thought that having beta thalassaemia trait may protect you against malaria.
Attribution: BUPA