They were partially correct. They do not use P-traps on the plumbing in China. That is the problem. The toilet forms a P-trap with the water in the bowl so that is ok.
The sink though has a straight drain hookup to the common sewage stack that it is hooked up to.
Someone flushes on a floor above when they have covid then aerosolized contaminated feces exit the sink drain in the apartments below and contaminate the general area of the sink where you brush your teeth,wash your face etc. Voila’ you eventually get infected.
This was figured out by two cases. One in Hong Kong and one in Shanghai.
I suspect it was similarly being spread by the flushing of pressure toilets, which in turn fed aerosols into the ventilation systems and increased propagation.
Thank you for the details! I knew it was something along those lines, but did not realize their system was different.
Makes perfect sense now.
This is why I love FR.....you can always learn something.