Oh my this is soooo gross and I experienced this first hand, a close relative of hubby's did that once in our place. I couldn't sleep I kept getting up and washing the kitchen sink. And even made hubby clean the sink. After that I made a point to always keep the kitchen door closed whenever that relative came over.
We wash our food in the kitchen sink, how can ppl spit in it. ewwww.... I also knew someone who would do brush her teeth and do her face cleansing in the kitchen sink.
Ok ok, how about moms taking babies soiled diapers off in your living room, throwing the diaper under your kictchen sink and washing their hands in the sink. Now that is a more possible scenario, innit?