Also, it might be incredibly rare because most people follow the instructions, and/or the people who follow instructions poorly live in areas with highly treate

Also, it might be incredibly rare because most people follow the instructions, and/or the people who follow instructions poorly live in areas with highly treated water (essentially the entire urban population). In warmer rural areas, using rainwater or borewater, it is likely to have significant growth of Naegleria fowleri. Children are particularly susceptible. https://healthywa.wa.gov.au/Articles/A_E/Amoebic-meningitis https://healthywa.wa.gov.au/Articles/A_E/Amoebic-meningitis https://www.cdc.gov/parasites/naegleria/pathogen.html https://www.cdc.gov/parasites/naegleria/pathogen.html https://www.abc.net.au/news/2018-03-20/professor-encouraging-rural-communities-to-treat-their-water/9567366 https://www.abc.net.au/news/2018-03-20/professor-encouraging... There is a similar problem where cleaning contact lenses with untreated water leaks to Acanthamoeba infection, leading to possible blindness of loss of an eye.