I applaud your inner skeptic, and your logic is persuasive. However, microbiologist Dr. Charles Gerba, member of the Department of Virology and Epidemiology at Baylor College of Medicine from 1974 to 1981, does make a strong, science-based appeal to reason concerning the transmission of pathogens through the environment. “Large numbers of bacteria and viruses when seeded into household toilets were shown to remain in the bowl after flushing,” Gerba’s research shows, “due to the adsorption of the organisms to the porcelain surfaces of the bowl, with gradual elution occurring after each flush.”
The “MythBusters” TV show tried and was unable to disprove that particles from a flushed toilet could travel to a toothbrush and, based on the research of Gerba and others, you may wish to reconsider leaving your toothbrush exposed. “The detection of bacteria and viruses falling out onto surfaces in bathrooms after flushing,” according to Gerba, “indicated that they remain airborne long enough to settle on surfaces throughout the bathroom. Thus, there is a possibility that a person may acquire an infection from an aerosol produced by a toilet.” (Emphasis added.) Source: Gerba CP, et al. Microbiological hazards of household toilets: droplet production and the fate of residual organisms. Appl Microbiol. 1975 Aug;30(2):229-37.
Researchers at Aston University, Department of Pharmaceutical and Biological Sciences, in the UK found that “large numbers of micro-organisms persisted on the toilet bowl surface and in the bowl water which were disseminated into the air by further flushes.” According to Dr. John Barker, the study’s lead author, “individuals may be unaware of the risk of air-borne dissemination of microbes when flushing the toilet and the consequent surface contamination that may spread infection within the household, via direct surface-to-hand-to-mouth contact. Some enteric viruses could persist in the air after toilet flushing and infection may be acquired after inhalation and swallowing.” Source: Barker J, et al. The potential spread of infection caused by aerosol contamination of surfaces after flushing a domestic toilet. J Appl Microbiol. 2005;99(2):339-47.
In another study, Barker found that, “Salmonella bacteria persisted in the biofilm material found under the recess of the toilet bowl rim, which was difficult to remove with household toilet cleaners. In two homes Salmonella bacteria became incorporated in the scaly biofilm adhering to the toilet bowl surface below the water line. Salmonella enteritidis persisted in one toilet for four weeks after the diarrhoea had stopped, despite the use of cleaning fluids…. The results suggest that during diarrhoeal illness, there is considerable risk of spread of Salmonella infection to other family members via the environment, including contaminated hands and surfaces in the toilet area.” Source: Barker J, et al. Survival of Salmonella in bathrooms and toilets in domestic homes following salmonellosis. J Appl Microbiol. 2000 Jul;89(1):137-44.