COVID-19 is once again in retreat. The reasons remain somewhat unclear. "This is as good as the world has looked in many months," Dr. Eric Topol of Scripps Research wrote. The most encouraging news is that the most serious forms of COVID are also declining. The number of Americans hospitalized with COVID has fallen about 25% since Sept. 1. Daily deaths ... have fallen 10% since Sept. 20. It is the first sustained decline in deaths since early summer. These declines are consistent with a pattern that readers will recognize: COVID's mysterious two-month cycle. Since the COVID virus began spreading in late 2019, cases have often surged for about two months sometimes because of a variant, such as delta " and then declined for about two months. Public health researchers do not understand why. Many popular explanations " such as seasonality or the ebbs and flows of mask wearing and social distancing " are clearly insufficient, if not wrong. The two-month cycle has occurred during different seasons of the year and occurred even when human behavior was not changing in obvious ways. The most-plausible explanations involve some combination of virus biology and social networks. Human behavior does play a role, with people often becoming more careful once caseloads begin to rise. But social distancing is not as important as public discussion of the virus often imagines. "We've ascribed far too much human authority over the virus," as Michael Osterholm, an infectious-disease expert ... has said.