Disease

she has a tumor, but it inexplicably goes away.

"she has a tumor, but it inexplicably goes away."

Medically speaking, that's a sound concept. There really exists such a phenomena, for which medical science has no definitive explanation.
It occurs very rarely, as the MDs in the film tell Melody, and it is believed that in these cases, the patient's immune system finally recognises the cancer as an intruder and attacks it: basically what the docs told Melody.

I haven't heard they've established that this cure is brought about by changing the patient's lifestyle.

But, let's not lose sight of the fact that it's only a film – and a very good one.