Thalassaemia can cause significant complications, including pneumonia, iron overload, bone deformities and cardiovascular illness. However, this inherited disease of red blood cells gives a degree of protection against malaria, which is or was common in the regions where the trait is common. This selective survival advantage for carriers (known as heterozygous advantage) is responsible for keeping the mutation in populations way above its mutation rate. Carriers are heterozygous for the thalassaemia allele, meaning only one of their two alleles is mutant (abnormal). There are a number of different versions of thalassaemia. Each one is caused by a mutation in a different position in the genome.