https://doi.org/10.3389/fimmu.2017.01263
"The paper here finds that persistent cytomegalovirus infection is likely the mediating mechanism linking early life stress and later increased risk of age-related disease, acting through accelerated immune system dysfunction. This implies that the early stress may or may not be all that important, as - for whatever reason - the groups selected as examples of stress in early life are also more likely to be infected. That might be the short-term detrimental effects of stress on immune function, or it could be a matter of being in close contact with more distinct groups of people during childhood, as is the case for the adopted individuals in the study here. Cytomegalovirus is a persistent herpesvirus that the immune system cannot effectively clear from the body. Near everyone becomes infected at some point in life, and extensive evidence links this infection with immune system dysfunction. Increasing numbers of immune cells become dedicated to uselessly fighting cytomegalovirus, and ever fewer are left for everything else the immune system must accomplish. Other than this long-term corrosion, cytomegalovirus doesn't cause obvious symptoms in the vast majority of people - few notice the initial infection."
This is also compatible as a mechanism to explain greater disease risk in descendants, since growing up in an infected household presumably raises the risk of early infection with cytomegalovirus.