When I was in 6th grade (2002-2003), I remember watching this 2D animated film on cells in the immune system. My guess is it was probably made in the 70s or 80s based on the style of animation (similar to Schoolhouse Rock).
In the video a narrator would describe the different cells in the Immune System like B Cells and Helper T Cells. This was done by personifying them as little anthropomorphized creatures. The Helper T Cell was like a soldier or something. There were also sections of the video that showed a cartoon boy (and these cells were living in his body). There was a section on cancer, who is shown as this little goblin creature who is multiplying on a (cell) wall. There was also a part where the boy is smoking a cigarette and it makes him green in the face.
I am 100% sure it was not "Once Upon A Time... Life". But is is very similar to that in how it personified the cells.