The normally rare infection, called mucormycosis, has a mortality rate of 50%, with some only saved by removing an eye or jaw bone.
But in recent months, India saw thousands of cases affecting recovered and recovering Covid-19 patients.
Doctors suspect there may be a link with the steroids used to treat Covid.
Diabetics are at particular risk, with doctors telling the BBC it seems to strike 12 to 15 days after recovery from Covid.
On Thursday, Health Ministry Joint Secretary Lav Agarwal wrote to India's 29 states to ask them to declare it an epidemic.