Water is a critical resource in hospitals, but seriously ill and immunocompromised patients are particularly at risk from infection, even from those microbes that are commonly present in water and pose little threat to the healthy. If a tap is used relatively infrequently, the water at the end of the pipe can be prone to stagnation, creating ideal conditions for the formation of a biofilm, allowing bacteria to multiply and making infection even more likely.
The two main waterborne bacterial threats are Pseudomonas aeruginosa and Legionella species. Both are fairly resistant to the chlorine and chloramines commonly used in water treatment and the disinfection of distribution systems, and in the relatively warm environment at the ends of pipes they will grow.
Legionella is usually transmitted in aerosol form, with shower heads and air conditioning units being common culprits. P. aeruginosa is more likely to be spread by direct contact with a tap or sink; if a patient or healthcare worker uses the tap and fails to disinfect their hands properly afterwards, transmission can result.