Peaks and troughs, lumps and slumps, wave after wave of surge and retreat: I have been following the ups and downs of this curve, day by day, for a year and a half. The graph records the number of newly reported cases of Covid-19 in the United States for each day from 21 January 2020 through 20 July 2021. That’s 547 days, and also exactly 18 months. The faint, slender vertical bars in the background give the raw daily numbers; the bold blue line is a seven-day trailing average. (In other words, the case count for each day is averaged with the counts for the six preceding days.)
I struggle to understand the large-scale undulations of that graph. If you had asked me a few years ago what a major epidemic might look like, I would have mumbled something about exponential growth and decay, and I might have sketched a curve like this one:
My imaginary epidemic is so much simpler than the real thing! The number of daily infections goes up, and then it comes down again. It doesn’t bounce around like a nervous stock market. It doesn’t have seasonal booms and busts.