“Mucormycosis will tail off and go back to baseline as the Covid cases subside,” said Dr. Dileep Mavalankar, an epidemiologist. “But it may come back in the third wave unless we find out why it is happening.” Many doctors in India think they know why. The bone-and-tissue-eating fungus can attack the gastrointestinal tract, the lungs, the skin and the sinuses, where it often spreads to the eye socket and the brain if left untreated. Treatment for the disease involves complex, often disfiguring surgery and an uncommon and expensive drug, contributing to a mortality rate above 50 percent. Mucormycosis is not passed from person to person. It develops from commonplace spores that sometimes build up in homes and hospitals. Doctors believe India’s crowded hospitals, and their dire lack of medical oxygen, left the fungus an opening.