Superbugs resistant to emergency antibiotics are spreading in hospitals, a Europe-wide study shows.
Drugs called carbapenems are used when an infection cannot be treated with anything else.
The spread of resistant Klebsiella pneumoniae was "extremely concerning", researchers from the Sanger Institute said.
And they warned other bugs could become resistant too - because of the unique way bacteria have sex.
What is Klebsiella pneumoniae?
It can live completely naturally in the intestines without causing problems for healthy people.
However, when the body is unwell, it can infect the lungs to cause pneumonia, and the blood, cuts in the skin and the lining of the brain to cause meningitis.
Is it becoming a problem?
Some strains are developing resistance to antibiotics.
"The alarming thing is these bacteria are resistant to one of the key last-line antibiotics," Dr Sophia David, from the Sanger Institute, told BBC News.
"The infections are associated with a high mortality rate.
"It's already worrying that we're seeing 2,000 deaths in 2015 - but the concern is that if action isn't taken, then this will continue to rise."
Deaths from carbapenem-resistant K. pneumoniae have gone up from 341 in Europe in 2007 to 2,094 by 2015.
What does the study show?
This is the largest study of carbapenem resistance in K. pneumoniae, with 244 hospitals involved from Ireland to Israel.
Researchers analysed the bacterium's DNA - its genetic code - from samples from infected patients.
"Our findings imply hospitals are the key facilitator of transmission [and suggest that] the bacteria are spreading from person-to-person primarily within hospitals," said Dr David.
"The fact that we see the same high-risk clones in many different hospitals around Europe also shows there's something special about those strains."
The results were published in Nature Microbiology.
How big a problem is this?
Drug-resistant K. pneumoniae could continue to spread or pass its resistance on to other species of bacteria.
Two bacteria can meet up and have bacterial sex - called conjugation - and a short string of genetic information, called a plasmid, is shared between them.
And the study found the instructions that give K. pneumoniae carbapenem resistance written on to plasmids.
"These have the ability to spread very rapidly through bacterial populations," said Dr David.
What can be done?
The best way to deal with drug-resistant infections is to avoid getting them in the first place.
"We are optimistic that with good hospital hygiene, which includes early identification and isolation of patients carrying these bacteria, we can not only delay the spread of these pathogens, but also successfully control them, said Prof Hajo Grundmann, from the University of Freiburg.
"This research emphasises the importance of infection control and ongoing genomic surveillance of antibiotic-resistant bacteria to ensure we detect new resistant strains early and act to combat the spread of antibiotic resistance."
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