People who get a copy of the gene from only one parent do not develop the disease, but they do develop immunity to most forms of malaria. In other words, the gene was (and in some places still is) useful, especially in sub-Saharan Africa.
The sickle cell anaemia disease is inherited only when the gene is inherited from both the parents. However, inheriting the gene from only one parents, provides immunity to malaria, which is just crazy provided that this is only seen in Africans!
