The ISS is a good example of a fully isolated environment. No new bacteria or viruses arrive there apart from spacecraft arrivals.

I've been curious for a while what human health would look like if there was a small group of people isolated for many decades. Would they effectively be disease free after the first few weeks?

As well as removing flu and colds, might it also reduce things like heart disease and Alzheimer's which we have weak evidence are linked to transmissible diseases?