Scientists are warning that levels of resistance to treatments for fungal infections are growing, which could lead to more outbreaks of disease.
Intensive-care and transplant patients and those with cancer are most at risk because their immune systems cannot fight off the infections.
Writing in Science, researchers said new treatments were urgently needed.
Fungal infections had some of the highest mortality rates of infectious diseases, an expert said.
An international team, led by researchers from Imperial College London and the University of Exeter, found a huge increase in resistance to antifungal drugs worldwide over the past 30-40 years.
Everywhere in the air
Prof Matthew Fisher, professor of epidemiology at Imperial College London, said this was probably down to farmers spraying their affected crops with the same drugs used to treat fungal infections in patients.
The "unintentional by-product of this 'dual use' of drugs in the field and the clinic" was that drugs were no longer working in patients who were unwell, he said.
"There are fungi in the air all the time, in every lung-full of air we breathe," Prof Fisher said.
"Bodies with a fully functioning immune system do an amazing job of curing the infection – but it can become an invasive fungal infection in others and [this] needs a drug."
He said the number of people at risk from fungal infections was rising rapidly as a result of increased numbers:
- people with HIV
- the elderly
- patients in hospital
The review said improvements were needed in how existing drugs were used, as well as an increased focus on the discovery of new treatments, in order to avoid a "global collapse" in the fight against fungal infections.
'Under the radar'
Prof Sarah Gurr, from the University of Exeter, said: "Emerging resistance to antifungal drugs has largely gone under the radar, but without intervention, fungal conditions affecting humans, animals and plants will become increasingly difficult to counteract."
Prof Gordon Brown, director of the Medical Research Council Centre for Medical Mycology, said some fungal infections had mortality rates of more than 50%.
He said: "Given the high rates of mortality of these infections, these disturbing trends suggest that even our limited ability to treat these diseases is being severely compromised."
Prof Brown said we were also seeing the rise of new multidrug-resistant fungi such as Candida auris.
Candida auris is responsible for increasing rates of invasive fungal infections in hospitals around the world – but there are very few treatments for it.
The review said it was resistant to all antifungal drugs and "presents a threat to intensive-care units" because it could survive normal efforts at decontamination.