I went to a single visit to a hospital once and after the treatment, I got 4 different bills. I was amused why I was getting bills from places I had no idea about. Then I realized each entity had a different billing department - the hospital's bill, the doctor's bill, the tester's bill, etc. all related to the single visit to the same hospital.
I had to call each of the 4 departments and deal with insurance. It was a nightmare!