Hospitals in India have seen a rise in cases of mucormycosis, a life-threatening infection.
Experts have linked steroids that treat COVID-19 to the uptick since they dampen the immune system.
One doctor in Mumbai said 11 of his patients had required life-saving eye-removal surgery.
In the wake of India's second wave of COVID-19, hospitals are reporting an uptick in cases of mucormycosis, or "black fungus" - a serious but rare fungal infection.
Dr. Akshay Nair, an eye surgeon who works at three hospitals in Mumbai, told the BBC he saw at least 40 patients with the fungal infection in April alone.
That's a huge uptick from the average number: He said he had seen only 10 cases of mucormycosis in the prior two years.
Most patients had diabetes and developed symptoms two weeks after recovering from COVID-19. Eleven of the patients had to have surgery to remove an eye to stop the spread of the infection, which is lethal in half of cases.
Nair told the BBC that doctors thought steroids, a treatment that has proved an effective treatment for severe COVID-19, might play a role in fueling the infections. While steroids reduce inflammation and can prevent lung damage in those with COVID-19, they also suppress the body's immune system.
"Diabetes lowers the body's immune defenses, coronavirus exacerbates it, and then steroids which help fight Covid-19 act like fuel to the fire," Nair told the BBC.
There is no data counting mucormycosis cases across the world, and Indian officials have said there is no major outbreak, according to Reuters.
One expert, however, said the numbers appeared to be higher in India, which has a high rate of diabetes, than in other countries.
"There have been cases reported in several other countries - including the UK, US, France, Austria, Brazil, and Mexico, but the volume is much bigger in India," David Denning, a professor at Manchester University, told Reuters. "And one of the reasons is lots and lots of diabetes, and lots of poorly controlled diabetes."
Indian government officials have told doctors to be on the lookout for symptoms of mucormycosis.
It is caused by coming into contact with fungal spores, which can be found in soil, plants, and manure.
According to the Centers for Disease Control and Prevention, early symptoms include:
One-sided facial swelling
Nasal or sinus congestion
Symptoms can escalate to:
Blackening of the nose
Blurred or double vision
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