Jazz has become a post modern artform these days, insofar as it takes an overview of all styles of music and is prepared and able to absorb any style into its repertoire.

John Mclaughlin was a typical example of this and his work in the realm of Jazz fusion resulted in the inclusion of Indian and flamenco scales and vocabulary (amongst others) into the jazz "dictionary".

In this respect I don't think Jazz and folk can be compared as While Jazz can derive interesting and useful ingredients from folk and remain Jazz, I'm not sure that folk can do the same in reverse without becoming folk fusion, in which case it is no longer folk but a sub-branch of Jazz.

Having said that, American folk is said to include many of the songs of Cole Porter, Gershwin etc, and their music also qualifies as Jazz music, whether performed exactly as intended or reinterpreted.

Generally I would argue that Jazz is distinct on the basis of its emphasis on original interpretation and spontenaiety, but even then the boundaries are blurred, as many folk songs are interpreted differently and written in pretty much the same format as many Jazz tunes and chord progressions are notated.

Both began as Aural traditions and in both cases, written scores are based on attempts to transcribe what the ear has heard, often wrongly.

There are vast grey areas, so much so that i would rather not give an opinion.