A very interesting bit of research that attempts to explain why pandemics of influenza don't just hit and then disappear instead of coming back with waves of infection hitting over different periods of time.
Quote from afludiary.blogspot.com
One of the great mysteries about influenza pandemics is why illness seems to come in waves, sometimes separated by months with little or no flu activity.
The 1918 pandemic appeared in many regions as three separate waves. A milder wave during the spring and summer of 1918, followed by an intense and very deadly wave in the fall, and a return of the following spring.
Again in 1957, and in 1968, we saw pandemic influenza come in repeated waves, but the exact reason why the virus should flare, infect some - but not all - then recede for months only to return, has never been adequately explained.
More at: Study: Prior Immunity And Pandemic Waves