The intensive-care units at the Salvador Zubirán hospital in Mexico City have been operating at full capacity for three straight months—their beds filled with unconscious Covid-19 patients, positioned face down and connected to ventilators. Every person who dies or recovers is replaced within hours by another who is critically ill and in need of life support. “It’s a nonstop flow. We have never seen anything like this before,” said Thierry Hernández, the head of emergency services at the Salvador Zubirán hospital, one of the…