The same genetic condition that causes sickle-cell anemia can also protect against:

The same genetic condition that causes sickle cell also protects against MALARIA. Sickle cell anemia is an inherited blood disease that causes red blood cells to become sickle shaped. While the disease is a serious illness and can be fatal, the trait only produces mild symptoms in some cases. Sickle cell trait is prolific in malaria ravaged areas because it is a natural protection against the mosquito borne disease. The sickle shape of the red blood cells inhibits the spread of the protozoa (the pathogen) that causes malaria.  

B. they all live in soil