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NYTimes |  Since the pandemic began, Covid has often followed a regular — if mysterious — cycle. In one country after another, the number of new cases has often surged for roughly two months before starting to fall. The Delta variant, despite its intense contagiousness, has followed this pattern.

After Delta took hold last winter in India, caseloads there rose sharply for slightly more than two months before plummeting at a nearly identical rate. In Britain, caseloads rose for almost exactly two months before peaking in July. In Indonesia, Thailand, France, Spain and several other countries, the Delta surge also lasted somewhere between 1.5 and 2.5 months.

And in the U.S. states where Delta first caused caseloads to rise, the cycle already appears to be on its downside. Case numbers in Arkansas, Florida, Louisiana, Mississippi and Missouri peaked in early or mid-August and have since been falling

We have asked experts about these two-month cycles, and they acknowledged that they could not explain it. “We still are really in the cave ages in terms of understanding how viruses emerge, how they spread, how they start and stop, why they do what they do,” Michael Osterholm, an epidemiologist at the University of Minnesota, said.

But two broad categories of explanation seem plausible, the experts say.

One involves the virus itself. Rather than spreading until it has reached every last person, perhaps it spreads in waves that happen to follow a similar timeline. How so? Some people may be especially susceptible to a variant like Delta, and once many of them have been exposed to it, the virus starts to recede — until a new variant causes the cycle to begin again (or until a population approaches herd immunity).

The second plausible explanation involves human behavior. People don’t circulate randomly through the world. They live in social clusters, Jennifer Nuzzo, a Johns Hopkins epidemiologist, points out. Perhaps the virus needs about two months to circulate through a typically sized cluster, infecting the most susceptible — and a new wave starts when people break out of their clusters, such as during a holiday. Alternately, people may follow cycles of taking more and then fewer Covid precautions, depending on their level of concern.

Whatever the reasons, the two-month cycle predated Delta. It has repeated itself several times in the U.S., including both last year and early this year, with the Alpha variant, which was centered in the upper Midwest