on the one hand, on the other hand

http://www.sciencedaily.com/releases/2006/11/061112094549.htm

Researchers at the University of Virginia School of Medicine and Federal University of Ceará in Brazil have joined forces to study if the gene believed to contribute to Alzheimer's protects children from the developmental stresses of early childhood diarrhea.

In addition, Guerrant says the research will also shed further light on situations where negative genetic traits are found to have beneficial effects that likely help to explain their presence in human evolution. An example of this phenomenon, called a balanced polymorphism, is sickle cell anemia, a genetic disease in which a double "dose" of the sickle cell gene causes red blood cells to form odd shapes and results in pain and anemia. A single "dose" of the same gene, however, makes a person resistant to malaria, a deadly tropical disease.