Back in November, Ajeet Jain felt like he was living a nightmare. The large public hospital where he works in India's capital was full of covid-19 patients, hundreds of them so ill they required intensive care. Three months later, the situation is unrecognizable. The number of coronavirus patients at the hospital can be counted on one hand. Out of 200 ventilators, only two are in use. Hospitals treating covid-19 patients around the country report similar experiences. "It's a big, big relief," Jain said. The apparent retreat of the coronavirus in India, the world's second-most populous nation, is a mystery that is crucial to the future course of the pandemic. Epidemiologists in India say that there is only one likely explanation for the decrease in new cases: The virus is finding it harder to spread because a significant proportion of the population, at least in cities, already has been infected. The results of a nationwide antibody survey ... indicated that more than 1 in 5 Indians about 270 million people " had been exposed to the virus as of early January. In major cities, infection rates are even higher. A recent study of 28,000 people in India's capital found 56 percent had coronavirus antibodies. By comparison, a study published last month estimated that more than 14 percent of the population in the United States had coronavirus antibodies as of mid-November. India has recorded 155,000 deaths, or about 112 per 1 million of population, compared with 1,362 per million in the United States.