word choice - "Being used in a disease" vs "being used on a disease"

The treatment didn’t work, because it was being used in/on a disease
that wasn’t a disease.

A native English speaker told me on was the correct choice.

But Google Books returns 0 results for used on a disease and 6 for used in a disease. So I'm confused. What's the correct usage and why?

Answer

I suggest you say: "treatment for a disease".

Ngram: for a disease vs in/on a disease.

• The treatment didn't work, because it was (being used) for a disease that was not a disease.