My reply is for can the coronavirus spread through pipes? Maybe it was spread in the entry way of the apartment building, maybe the person infected had just entered the building & sneezed as they walked to the stairs or elevator then the person that caught it walked in to the entry way & breathed in the germs , or maybe they used the same elevator or stairs. I would think that would be more likely to have happened their paths crossed some how as the infected person coughed &/or sneezed.
Waste plumbing in large blocks of flats does not keep the different types of waste water separate. If the traps are opened air from the sewage can be sucked straight into a room. Sink and toilet traps are normally U or S bends which give little trouble but wet room traps are very different. Many types of wet room traps can be easily removed by hand and often are when the shower is being used. The combination of powerful extractor fans and removed wet room trap would be ideal conditions for sucking infected air drops into a dwelling. In my flat switching on the kitchen extractor creates such a large air flow through the bathroom that doors are blown shut!