Pseudomonas aeruginosa, a bacteria that—like Legionella —lives in water pipes as biofilm builds up, can be deadly for immunocompromised patients.

The deaths of two premature infants at Miami Children’s Hospital illustrates the risk, although in that case, the source of the Pseudomonas is unclear.

What is more certain is that hospitals can’t count on chlorine piped into local water supplies to wipe out Pseudomonas because the bacteria can resist the chemical, says Matthew Freije, president of HC Information Sources, Inc., a waterborne pathogens consulting company in Carlsbad, CA.