India has ordered tighter surveillance of a rare fungal disease hitting Covid-19 patients, officials said on Thursday, piling pressure on hospitals struggling with the world's highest number of daily infections of the novel coronavirus.

Mucormycosis, or "black fungus", usually infects people whose immune system has been compromised, causing blackening or discolouration over the nose, blurred or double vision, chest pain, breathing difficulty, and coughing blood.

Doctors believe the use of...