In the beginning, many suspected that Covid was being spread via sewer gasses.
In China apartment buildings, residents were locked in their apartments with zero outside contact, but were still contracting Covid.
They suspected that the interconnected sewer vent lines were transporting the virus from apartment to apartment via drain lines, then out the vent at the top of the building.
It seems the best tracing has been wastewater.
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.