If you already have immunity to a bug, being isolated will not diminish that immunity. You will already have the antibodies to defend against that bug.
They don't go away if you don't get exposed again.
If you do not have immunity, you will still not have it, until you are exposed to the bug, and your immune system reacts to it.
And, if you are immune, you can still carry the bug around with you if exposed, and pass it on to others who are not immune. Your immune system will not react to bugs on you clothes or skin, and it can take a day or so for the immune response to kill off any bugs you get in you nose, or mouth or on your skin. So you can still pass it on for the better part of a day, even if you are immune.
Isolation still works better that communal gathering. I have seen this bug in action, and it is definitely worse than a "Bad Cold"