Re: testing.
Just to give a perspective from a real case.
Three members of my family (a whole household - one of them a nurse) contracted Covid19 in September. Happily all got through it.
One of them (male) never tested positive. He had three tests and despite being unwell with all the symptons, all tests were negative (the other two householders both tested positive).
An anti-bodies test subsequently "confirmed" he had had Covid19 all along (which was pretty obvious anyway).
Make of that whatever you will.