I've been able to smell infections (which makes sense, the immune response of an infection would mis with your whole microbiome). That's pretty different from smelling Parkinson's.
There seems to be a "gut-brain axis" at work, to the point where some are starting to think the gut is the starting point.
We're seeing an increasing number of disorders -- psoriasis [2], lupus, schizophrenia, among others -- where there's an underlying connection to the gut microbiome and to inflammation in general caused by "pathobionts" [3].
[1] https://parkinsonsnewstoday.com/2019/02/12/understanding-gut...
[2] https://www.sciencedirect.com/science/article/pii/S0738081X1...