In the 2003 SARS outbreak,the biggest non-hospital outbreak in Hong Kong was at Amoy Gardens, a complex of eight-story apartment towers, and particularly in one of the towers. Much speculation involved how the virus, borne by various bodily fluids and by aerosols (think coughing or sneezing), spread quickly among residents, living on various floors and without much apparent contact.
What seems to have occurred involves diarrhea, experienced by most SARS victims, which involves many trips to the toilet, and much flushing. A crack in a pipe in the building's air-shaft produced a fine mist of virus bearing particles, available to anyone accessing the air-shaft. Perhaps more significantly, the bathrooms in the tower were equipped with floor drains, with P-traps, many of which seem to have been dry. (The P-trap under a floor drain is similar to the one in the drain under your sink - it's designed to hold water, so as to provide a barrier between sewer and your bathroom.) Without water in the P-trap, the air in your bathroom or kitchen connects directly to the air in the sewer and the vent pipe.
The thinking is that each flush produced an aerosol plume, which traveled up the vent pipe into any other floor drains in the vicinity that had dry P-traps.
(For more details, check Karl Taro Greenfield's China Syndrome. Fascinating, if chilling read; I picked it up from an interest in China and pandemics, and stumbled across the P-trap bit.)
Now, in providing commercial janitorial service, we emphasize health; we're careful to train our folks to pour a bit of water (6 or 8 ounces is adequate), once per week, into any seldom used drains (think floor drains, common in commercial restrooms, or the slop sink back in the shop, or the drain in the never used (I hope) eyewash station in the manufacturing area. Filling the P-trap weekly seems adequate to counteract evaporation.
We talk about filling the P-trap simply to prevent odors from the sewer from entering restrooms and other areas. I see the occasional note in janitorial industry trade journals about filling P-traps, to prevent odors. I don't believe I have ever seen anything about the potential of disease transmission through dry P-traps.
Are we missing something here?