CNN tells me something ominous and surprising:

In each of the four major pandemics since 1889, a spring wave of relatively mild illness was followed by a second wave, a few months later, of a much more virulent disease. This was true in 1889, 1957, 1968 and in the catastrophic flu outbreak of 1918, which sickened an estimated third of the world’s population and killed, conservatively, 50 million people.

Via MR.