They Have Separate Immune Systems
Despite sharing many of the same major organs, Abby and Brittany have separate immune systems. One of them can be totally healthy while the other one is horribly sick. In fact, Brittany has had pneumonia twice in her life and Abby has never had it.
The twins both agree that being sick is one of the few times they wish they were separated. When Brittany was ill as a child, Abby remembers wishing to be separated after being bored and restless while confined in the same bed as her sick sister. Brittany became so upset by the thought she cried uncontrollably until Abby assured her she wouldn’t leave her side.