Cases of mucormycosis, a fungal infection that causes blackening or discolouration over the nose, blurred or double vision, chest pain, breathing difficulties and coughing blood, have been growing under India's second wave of Covid-19.
Doctors have told The Daily Telegraph that the Indian variant - also known as Delta - is causing unprecedented damage to the pancreas of otherwise healthy people, triggering sudden onset diabetes and soaring blood glucose levels - in turn allowing the fungus to thrive.
It comes as a Public Health England report suggested the dominant Delta strain is more likely to lead to people needing hospital treatment than the Alpha variant first detected in Kent last autumn.
Over the past week, The Daily Telegraph visited 10 hospitals across the western Indian state of Maharashtra, where doctors are treating some of the 11,000 patients who have now been struck down by the aggressive infection after recovering from Covid-19.
The sudden explosion in cases during India's brutal second wave was initially blamed on overprescription of steroids during Covid-19 treatment - which can provide a cheap, effective treatment to combat severe cases, but also suppress the immune system.
Prior to Covid-19, Seven Star Hospital would only see several mucormycosis cases in a year. But since mid-March, Dr Shailesh Kothalkar, the hospital's leading ear, nose, and throat (ENT) surgeon, has treated more than 280 patients and operated on 151 people, during gruelling 18-hour days at work.
"This new variant is causing damage to the beta cells in the pancreas, which produce insulin and regulate blood glucose levels," says Kothalkar.
"We need more investigation into this ... around 40 per cent more patients are developing diabetes after having Covid-19 during this second wave."
Across Maharashtra, exhausted doctors warn that - while long-term exposure to steroids was a contributory factor in some patients - mucormycosis cases among diabetes patients exploded only after the Delta variant emerged.
Children as young as 12 are known to have developed symptoms, while some of those admitted had blood glucose levels as high as 750mg/dl.
"I had seen 45 patients in my entire career and now we have had 66 patients admitted in just one month. Over the last five days, we've barely slept, we've done 40 surgeries," explains Dr Shrinivas Chavan, the head of the ENT department at Mumbai's government-run Grant Medical College and Sir JJ Hospital.
Prior to March 1, there had been just 41 recorded cases of coronavirus- induced mucormycosis worldwide. But, over the last week, there has been a spate of cases recorded in India's neighbours, including Nepal and Bangladesh, where Delta has also become dominant.
In an overloaded health system, monitoring of sugar levels is likely to be less stringent, putting patients at increased risk of the feared "black fungus".
In the UK similar scenes to India are unlikely, says David Denning, a professor of infectious diseases at the University of Manchester and chief executive of the Global Action Fund for Fungal Infections, though patients with poorly controlled diabetes may be at risk.
Mucormycosis is caused by a mucor commonly found in soils and decaying vegetables that infect the sinuses and lungs of an immuno-compromised person, before spreading to the brain.
Doctors say early detection, including regular blood glucose screening among recovered Covid-19 patients, is critical to stem the march of the fungus. But if it is allowed to progress for even several days, then the fatality rate can be as high as 50 per cent.
In the Seven Star Hospital in the western city of Nagpur, ward after ward is filled with patients who had recovered from Covid-19 yet - despite being previously healthy - were hit by mucormycosis symptoms.
Vikram Trivedi, 38, is one of those. He now needs help from three nurses just to walk.
Five days ago he had his left eyeball, maxillary sinus and the roof of his mouth removed to save his life from the fungus.
At the same hospital, Jijabai Thakare was wheeled into the emergency room by her two sons shortly before The Telegraph left.
The 63-year-old reached Nagpur from her remote village of Paramboh three days after developing symptoms.
Yet already, her left side was paralysed, indicating late-stage mucormycosis. Days later, Thakare died.