This leads me to believe it is either something that you are all frequently ingesting, whether through diet or normal breathing, or it is
something that is
dormant in your
cells. As it is
a common trait between you all, and something that is dormant is not this widespread, I would opt for the former explanation, that it is something that you all ingest regularly.
If you all spend the majority of your time indoors and/or in the same building, I might be able to suggest something.