For untreated flushing cistern, water became stagnant with high portion of sludge in it, this will be favorable for bacteria growth and may turn into epidemic spreading channel which jeopardize users health.
Common virus found in flushing water as below:

 

頭條新聞 – 大廈U形聚水管恐傳H5N1

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When flushing, water force creates huge amount of spray which is invisible that polluted toilet seat and the surrounded area

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