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Flushing a toilet produces hundreds of thousands of tiny droplets containing viruses and bacteria (source):

Up to 145,000 droplets were produced per flush, with the high-energy flushometer producing over three times as many as the lower energy [pressure-assisted high efficiency toilets] and over 12 times as many as the lowest energy [high efficiency toilets] despite similar flush volumes.

Bacteria and viruses can be transfered in those droplets:

Experimental work has clearly shown that droplet nuclei toilet plume aerosols are capable of entraining microorganisms as large as bacteria [...]

Additionally:

Both the vomit and feces of some infected persons may contain extremely high pathogen loads—concentrations of 105–109 Shigella (Thompson 1955), 104–108 Salmonella (Thompson 1955), and 108–109 norovirus (Atmar et al. 2008) per gram of stool and at least 106 norovirus per milliliter of vomit (Caul 1994) have been reported.

Similarly, sewage water produces droplets and was proposed as the transmission mode in the SARS outbreak in Hong Kong in 2003:

[...] airborne transport of the SARS coronavirus by such sewage-related bioaerosols was proposed as the likely disease transmission mode [...]


Apart from viruses and bacteria being transfered through toilet and sewage aerosol, can human parasite eggs be transfered as well given the droplet size (99% of droplets in the linked research sized <5μm in diameter and 95% <2μm)?

I'm interested in the aerosol, not the relatively large droplets that land on the toilet itself.

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Common small protozoa that are present in infective loads in stool have not been documented in toilet aerosols. The size of the organism is going to be an issue here. As you can see from this table from the CDC, intestinal parasites (and ova) of concern to human health are larger than most droplets. Compare, E. coli and Shigella (0.5 - ~2 $\mu m$). Salmonella tends to be a litter larger (up to 5$\mu m$), but I expect those 5 $\mu m$ specimens are not the ones transmitted through aerosols. Cryptosporidium would probably be the most likely candidate, and that's one shown in the first linked study to not be present in aerosols. I wouldn't say it's impossible, but certainly transmission through aerosolized particles is less likely than for the smaller viral and bacterial organisms.

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