Health experts in India are seeking to find out what caused a major drop in coronavirus cases in recent months. New infections had climbed for months across India last year, but then began falling in September. Now the country is reporting about 11,000 new cases a day. That is down from nearly 100,000 during the worst period of the crisis. Experts studying the virus have suggested many possible explanations for the sudden drop, which happened in nearly every part of India. Two possible explanations are that some areas of the country may have reached “herd immunity” or that Indians may have some preexisting protection from the coronavirus. The term herd immunity describes a situation in which enough people in a population are resistant to a disease so that it cannot spread. The Indian government has said it believes the decrease happened, at least partly, because of mask-wearing across the country.