As a rule, high-ranking public-health officials try to avoid apocalyptic descriptors. So it was worrying to hear Thomas Frieden and Sally Davies warn of a coming health “nightmare” and a “catastrophic threat” within a few days of each other in March.

The agency heads were talking about the soaring increase in a little-known class of antibiotic-resistant bacteria: carbapenem-resistant Enterobacteriaceae (CREs). Davies, the United Kingdom's chief medical officer, described CREs as a risk as serious as terrorism (see Nature 495, 141; 2013). “We have a very serious problem, and we need to sound an alarm,” said Frieden, director of the US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia.

Their dire phrasing was warranted. CREs cause bladder, lung and blood infections that can spiral into life-threatening septic shock. They evade the action of almost all antibiotics — including the carbapenems, which are considered drugs of last resort — and they kill up to half of all patients who contract them. In the United States, these bacteria have been found in 4% of all hospitals and 18% of those that offer long-term critical care. And an analysis carried out in the United Kingdom predicts that if antibiotics become ineffective, everyday operations such as hip replacements could end in death for as many as one in six.