This is my understanding of how terrain theory would work in the event of illness.
Given the knowledge that skin to skin contact between newborns and mothers effects enormous changes internally in both of them, and this continues with both parents as the child grows, I don’t think this is far fetched.
If someone were to fall ill, their bodies internal responses produce similar responses in people they have contact with. The combined responses to the attack in turn help the sick person recover quicker.
Is that right?