Dangerous biofilms are easily created in the U-bend of the sink and are considered to be the perfect environment for hundreds of lethal bacteria to thrive.
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‘Sink and bath drains and plugholes, within both the hospital and a domestic setting, are troubling reservoirs for microbes.’
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Salmonella could get into the drain while washing chicken or E.coli could build-up from ground beef. Hair and dead skin could give rise to Fusarium solani, which could cause permanent vision damage. 
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While urinary tract infections are also a possibility, as some studies have detected Enterobacter cloacae in water pipes connecting to the plughole.
If left untreated, these can harbour E. coli and salmonella, which can then become airborne and pass up through the plughole into the home. This can compromise our immune systems and could potentially be life-threatening. However it usually results in diarrhoea.
Only a trickle of water is needed to then allow the bugs to make their way into sinks and bathtubs, where they can infect humans.
Source-Medindia