Which one of the following conditions though harmful in itself, is also a potential saviour from a mosquito borne infectious disease :

Sickle cell anaemia is an autosomal recessive disorder caused by a point mutation (GAG to GUG) that replaces glutamic acid with valine at the sixth position of the $\beta$-globin chain.
Homozygotes ($Hb^SHb^S$) suffer from severe anaemia. Heterozygotes ($Hb^AHb^S$, sickle cell trait) are mostly healthy.
The malarial parasite Plasmodium falciparum, spread by the Anopheles mosquito, cannot multiply well in red cells containing HbS, which sickle and are removed when infected. So heterozygotes are resistant to malaria.
This is why the sickle cell allele is common in malaria-prone regions such as tropical Africa (balanced polymorphism).
So sickle cell anaemia, though harmful, is a potential saviour from malaria.