What are the smallest functional units of life?

Question is too open or broad. Smallest units of life that can exist as a single entity or just exist period. You have a virus that exist but needs a host and cannot exist seperately. Prions are the smallest, except that virus and prions both are not units of life that can exist separately from their hosts. Or at least they can't reproduce separately from host organisms.

Similarly, the mitochondria in our own cells have their own DNA and reproduce independently of the cells they live in, so arguably those are pretty small units of life; again, they can't exist without host organisms.

If you require reproduction without other life, then bacteria are as small as it gets, if not it's prions, as far as we know.