Resistance to antibiotics poses a "major global threat" to public health, says a new report by the World Health Organization (WHO).
It analysed data from 114 countries and said resistance was happening now "in every region of the world".
It described a "post-antibiotic era", where people die from simple infections that have been treatable for decades.
There were likely to be "devastating" implications unless "significant" action was taken urgently, it added.
The report focused on seven different bacteria responsible for common serious diseases such as pneumonia, diarrhoea and blood infections.
It suggested two key antibiotics no longer work in more than half of people being treated in some countries.
One of them - carbapenem - is a so-called "last-resort" drug used to treat people with life-threatening infections such as pneumonia, bloodstream infections, and infections in newborns, caused by the bacteria K.pneumoniae.
Bacteria naturally mutate to eventually become immune to antibiotics, but the misuse of these drugs - such as doctors over-prescribing them and patients failing to finish courses - means it is happening much faster than expected.
The WHO says more new antibiotics need to be developed, while governments and individuals should take steps to slow the process of growing resistance.