In 1983 newborn identical twin boys were infected with HIV through a blood transfusion. Years later one twin was much healthier than the other. Amanda Carpenter explains how this is possible:
Once out of the womb, these twins no longer exist in identical environments. They are exposed to different pathogens, bacteria, and microbes, all of which affect the make-up of the immune system. The healthier twin’s immune system may be better able to fight the virus, and so the virus must grow, diversify, and recombine in order to propagate the infection. In other words, because the sicker twin has a more depressed immune system, the virus is replicating with less resistance, and there is less incentive for the virus to evolve. Divergent viral evolution in the case of these monozygotic twins is likely due to random mutation and recombination events, combined with antiviral pressure from the hosts, whose immune systems are not identical at all.
The Dish 