The patient could possibly have contracted some sort of foreign disease or might simply have an intolerance to a certain protein such as gluten with celiacs disease.
I would personally choose the lets bake a model

The Energy required for photosynthesis is absorbed by plants through green pigments known as Chlorophyll, in the form of Sunlight which is later used up during photosynthesis to produce Glucose.

I believe the answer you're looking for is photosynthesis, where plants release oxygen and humans use this to release carbon dioxide to the plants.