In a growing epidemic of the disease, India has recorded more than 8,800 cases of deadly "black fungus", reports the BBC.
Black fungus, also known as mucormycosis, is normally a rare infection which has a mortality rate of 50%, with some only saved by removing an eye, it said.
In recent months, thousands of cases involving recovered and recovering Covid-19 patients have been recorded in India.
Doctors say there is a link with the steroids used to treat Covid while diabetics are at particular risk. The fungus seems to strike 12 to 18 days after recovery from Covid, according to the report.
More than half of the cases have been registered in Gujarat and Maharashtra in western India, it said before adding at least 15 more states have confirmed cases ranging from eight to 900.
The disease has been declared an outbreak in India's 29 states due to an increase in cases.
Also read - What is 'black fungus' and why India?
According to doctors, newly opened wards to treat patients suffering from the disease across the country are quickly filling up.
The number of patients at the state-run Maharaja Yeshwantrao Hospital in Indore, which has 1,100 beds, had risen from eight a week ago to 185 on Saturday evening.
More than 80% of the patients need surgery immediately, Dr VP Pandey, head of the hospital's department of medicine, told the BBC.
"This surge in patients was definitely unexpected.
“We used to see one or two cases a year previously," he said.
He estimated that at least 400 people in Indore were infected with the disease.
"The black fungus infection has now become more challenging than Covid-19. If patients are not treated in time and properly, then the mortality rate can go up to 94%. The cost of treatment is expensive, and the drugs are in [short supply]," the doctor added.
Amphotericin B, also known as "ampho-B," is an antifungal intravenous injection that must be given every day for up to eight weeks to patients with mucormycosis, according to doctors.
The bulk of the patients were male and had recovered from Covid-19. The majority of them had been given steroids and all of them had underlying illnesses, the most common of which was diabetes, says the BBC report.
Four other Indian doctors looked at more than 100 cases of Covid-19 patients who had developed mucormycosis in a separate report. It was discovered that 79 of them were men, and 83 of them had diabetes.
Another study of 45 black fungus patients in two Mumbai hospitals found that all were diabetics or diagnosed with diabetes on admission. They all had very steep blood sugar levels.
What is mucormycosis?
Mucormycosis is a relatively uncommon infection. Mucor mould, which is commonly found in dirt, seeds, manure, and rotting fruits and vegetables, causes it.
"It is ubiquitous and found in soil and air and even in the nose and mucus of healthy people," Dr Akshay Nair, a Mumbai-based eye surgeon, told the BBC.
It affects the sinuses, the brain, and the lungs, and it can be fatal in diabetics or people who are seriously immune-compromised, such as cancer patients or people with HIV-Aids.