The systemic nature of the disease is now built into its official definition.
Could anyone please tell me what does the underlined part mean?
"Built in" is a strange verb to use there. OK, so some disease previously defined as a skin disease, or a disease limited to one organ, has been found to be systemic. They have changed the official definition of the disease to reflect this new knowledge, and it is now known as a systemic disease. "Built in!"