Normally, Klebsiella bacteria can be found in humans’ mouths, intestines and skin, and they cause no harm to people with healthy immune systems. But certain strains, like Klebsiella pneumoniae, can be dangerous for some people with weakened immune systems, particularly those in hospitals.

One strain of the bacteria is also resistant to a number of antibiotics, making the infection hard to treat. This type of Klebsiella pneumoniae produces an enzyme known as carbapenemase, which prevents antibiotics called carbapenems from killing the bacteria and treating the infection.