DON’T THEY ALWAYS: The Next Flu Pandemic Will Appear When You Least Expect It.

If a new flu pandemic emerges, it may be easy to spot. The epidemic is most likely to appear in spring or summer, researchers have found — not in the midwinter depths of the flu season.

Normally flu strikes in winter, when children are crowded into classrooms and the air is cold and dry — ideal for transmitting the influenza virus. But historically, that has not been true of the great flu epidemics.

A half-dozen flu pandemics — including those of 1889, 1918 (the Spanish Flu) and 2009 (the swine flu) — were all first detected between late March and late July, according to a study published recently in PLOS Computational Biology by researchers at the University of Texas at Austin and the Institute for Disease Modeling in Bellevue, Wash.

A person infected with the flu appears to be protected briefly against infection with any other flu virus, even genetically different ones, said Spencer J. Fox, a graduate student in infectious disease modeling at the University of Texas at Austin and an author of the paper.

That protection lasts about six weeks, Mr. Fox said. Even a highly infectious flu virus would be temporarily stymied in winter if the seasonal flu had already infected much of the population.

So maybe if a nasty flu pandemic is coming, we should deliberately infect ourselves with ordinary flu.