A recent Stanford University twins study concluded that environmental forces are more influential than genes when it comes to determining the make-up of an individual’s immune system. The study, led by Stanford immunologist Mark Davis, compared 210 identical and fraternal twins between 8 and 82 years old.

After analyzing blood samples that tracked 200+ immune system parameters including cell population frequencies, cytokine responses, and serum proteins, researchers found that in three-quarters of the measurements, differences between pairs of twins were strongly linked to non-heritable factors such as vaccinations, diet, and previous infections. Notably, a key environmental cause of immune system variation was the presence cytomegalovirus (“CMV”).

CMV is a common chronic infection affecting three in five Americans, and while usually harmless, CMV can pose a health threat to immune-compromised individuals and babies infected congenitally. Once CMV is in a person’s body, it remains for life.