fundamental, combined, coexistent disease (illness)

Hello,

My question is probably too specific, but I hope that you mightb have heard the terms I'm going to ask about and will recognize at least some of them:

1) When a person has several diseases, but one disease is the most serious and is treated most intensively, can we call this disease "fundamental disease/illness"?

2) When a person has several diseases, but none leads to death separately - only together they can result in death or other serious consequences. Is it "combined disease"?

3) Concerning 1) - if there is the main disease, are others called "coexistent diseases"?

Thanks

Because these questions are about medical jargon I have consulted my daughter and her husband, who are physicians. They both preferred principal disease to fundamental disease. And they both thought that the best word for multiple diseases is comorbidities, regardless of whether there is one which is the principal disease.