My sister and brother in law have been exposed to COVID-19 positive patients numerous times, but got tested for antibodies last Monday and were negative.

My brother got very sick after attending the World of Concrete show in Los Vegas. He got tested for antibodies Tuesday and was negative.

The version of Influenza going around is itself quite bad this year.

In a previous post in this thread I gave a link to a report from Stanford U medical center that found that about 20% of COVID-19 patients were co-infected at the same timewith either Influenza, RSV virus, or another virus.