How natural selection both helps and hurts.
Malaria and how it evolves along side us.
In areas of the world where Malaria is prevalent people are starting to develop sickle-cell anemia so that the persons with such cells would be less likely to catch malaria, yet as some may know, if all red blood cells are sickle-cell anemic it can cause several problems.
Sickle-Cell Anemia is the occurrence of either all or some of your red blood cells turning into a sickle or crescent moon form. When only some of the red blood cells are sickle-cell anemic you do not have any side effects other than a resistance to Malaria, but if all of your red blood cells are sickle cell anemic you could risk side effects such as episodes of pain, acute chest syndrome, infections, and anemia episodes.
Pictures of a Malaria insect, formation from normal cells to sickle cells, and sickle-cell origins
As you can see natural selection has it's back and forth, in this case we have adapted to Malaria with the sickle-cell trait. The downside of this being the severe and sporadic symptoms such as the ones I have listed above. Natural selection affects not only us, but everything around us as well, so as we adapt to environments other things do as well.