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. Wearing a mask in public is a rule in India and is strongly enforced in some cities. But experts have noted that virus cases also dropped in areas where mask-wearing was less common.

    Finding out causes for the drop could help officials better control COVID-19, the disease caused by the coronavirus. India has reported nearly 11 million COVID-19 cases and over 155,000 deaths.

    Dr. Shahid Jameel studies viruses at India’s Ashoka University. “If we don’t know the reason, you could unknowingly be doing things that could lead to a flare-up,” he told The Associated Press (AP).

    India, like other countries, misses many infections, and there have been questions about how it counts its virus deaths. But pressure on the country’s hospitals also dropped in recent months. This suggests that the virus’ spread is slowing.

    Recorded cases reached 9 million in November. At that time, official information showed that nearly 90 percent of all critical care beds with ventilators in New Delhi were full. This week, just 16 percent of those beds were occupied.

    Experts believe the drop cannot be the result of COVID-19 vaccines because India only began giving the injections in January. They say the effects of the vaccination program will only be seen later.

    Vineeta Bal studies how the body fights disease at India’s National Institute of Immunology. She told the AP she thinks the drop in virus cases could be related to large areas reaching herd immunity.

    But other experts have warned that even if herd immunity in some places is partly responsible for the drop, the population as a whole remains vulnerable. This means people must continue to take measures to protect themselves.

    New research suggests that people who got sick with one form of the virus may be able to get infected again with a new version. For example, Bal pointed to a recent study in Manaus, Brazil. It estimated that over 75 percent of people there had antibodies for the virus in October -- before cases saw a sharp rise again in January. “I don’t think anyone has the final answer,” she said.

    Experts say another possibility is that many Indians get exposed to a series of diseases throughout their lives. That could help build up immunity against new viruses. Among diseases common in India are cholera, typhoid and tuberculosis.

    “If the COVID virus can be controlled in the nose and throat, before it reaches the lungs, it doesn’t become as serious,” said Dr. Jameel of Ashoka University. “Innate immunity works at this level, by trying to reduce the viral infection and stop it from getting to the lungs,” he added.

    Even with the major drop in infections, Indian health officials are worried about new virus versions, or variants, slowing efforts to fight the coronavirus. Scientists have identified several new variants in the country, including some blamed for causing new infections in people who already had an earlier virus version.

    I’m Bryan Lynn.