Same way that people with sickle-cell anaemia still exist: being a carrier of the ailment protected you from other ailments (malaria in this case), and so it was never selected out of the gene pool by Nature

Replies

Makes sense, now, in a way it couldn't have when I wrote what you responded to. Thank you!

(See also: the wool-covered blinders I too often feel too comfortable pulling tightly over my eyes.)