An interesting case I was involved with over the past week illustrates the ubiquity of microorganisms and their ability exploit to any weakness to cause infection.
A guy falls off a ladder, tears his meniscus, and has surgery. Pretty routine until he develops a cystic mass at the surgical site. It's excised and grows Mycobacterium kansasii, a bacterium which is related to tuberculosis and merits aggressive treatment.
How does one get such an organism at a routine surgical site? Tap water.
Of course, not everyone's wound gets infected by this relatively rare bacterium so there is likely a large amount of host susceptibility at play. In fact, M.kansasii really gained prominence during the early years of the HIV/AIDS pandemic. While untreated HIV/AIDS causes catastrophic immune collapse, subtle alterations in an individual's immune repertoire are likely all that is needed when a the bacterium comes into contact with a hospitable wound.
The fact that such a pathogen flourishes in tap water reinforces the fact that we live in a world populated by microbes and that sterility is only relative. So, when a new mother lunges after her toddler when he drops his binky so she can wash it in the sink her activity just replaces one set of microbes with another.