A new study focuses on an unusual case involving identical twin baby boys who received a tainted transfusion and became HIV-positive a few years ago.

Today, one of the twins has a near-normal immune system and pretty good health, while the other has experienced numerous complications and is five years behind his brother on the growth chart.

BYU Biology Department chairman Keith Crandall is leading the effort to try to determine why the two responded differently.

Crandall says one theory holds that natural selection is the driving force, so the results should be similar. He says the other asserts that random genetics plays a role and therefore it cannot be predicted.