London, July 21: More than 4,300 people have died of the deadly "black fungus" in India in a growing epidemic of the disease.

India has reported 45,374 cases of this rare and dangerous infection, called mucormycosis, Health Minister Mansukh Mandaviya has said.

Nearly half of them are still receiving treatment. This aggressive infection affects the nose, eyes and sometimes the brain. Most of the cases involve recovered and recovering Covid-19 patients.

Doctors say the fungus has a link with the steroids used to treat Covid and diabetics are at particular risk.

The infection affects the sinuses, the brain and the lungs and can be life-threatening in diabetic or severely immunocompromised individuals, such as cancer patients or people with HIV/Aids. Doctors say the infection seems to strike 12 to 18 days after recovery from Covid.

Maharashtra, Gujarat, Tamil Nadu and Rajasthan have reported the most number of cases according to official data. Two states - Maharashtra and Gujarat - accounted for 1,785 deaths.

Dr Raghuraj Hegde, a Bangalore-based eye surgeon who has treated a number of mucormycosis patients, told the BBC that there had been "massive undercounting of both cases and deaths" of the disease. "Typically, deaths in mucormycosis occurs weeks to months after getting the diseases. Our present systems are not good to capture that data," he said.

'Cases were being also undercounted because the diagnosis was difficult in smaller hospitals and in rural areas and only a fraction of the cases reached hospitals in big cities,' he added.

Doctors said that many patients had died of the disease even before reaching a hospital and a number of treated and recovered patients appeared to be suffering from a relapse.

"We are seeing patients who were treated aggressively for the disease and discharged from hospitals returning with a recurrent infection which is manifesting in a wider spread of the disease in the eye or brain," Dr Akshay Nair, a Mumbai-based eye surgeon, told the BBC.

Steroids reduce inflammation in the lungs for Covid-19 and appear to help stop some of the damage that can happen when the body's immune system goes into overdrive to fight off the coronavirus.

But they also reduce immunity and push up blood sugar levels in both diabetics and non-diabetic Covid-19 patients.

It's thought that this drop in immunity could be triggering cases of mucormycosis. An anti-fungal injection is the only drug effective against the disease, doctors say.