This is a useless bit of pedantry. The exponential portion of the sigmoid curve is the only part that matters to human health, survival, and economic well-being.

No it's absolutely not!!

Epidemics are (more) properly modelled by the Gompertz function[1], and the position of the "transition" between the exponential portion and non-exponential points it the important point.

That is exactly what the person you are replying to is saying ("they always break into some other curve at some point.").

[1] https://en.wikipedia.org/wiki/Gompertz_function

Is the Gompertz function the function behind the standard "epi curve"? It appears from the CDC web site and other places that the epi curve is the standard way epidemics are modeled. The CDC even has a simple tutorial for creating them: https://www.cdc.gov/training/quicklearns/createepi/index.htm...

No.

An epi curve is created from observations and shows new infections per day.

The Gompertz function is a mathematical formula and can be used to estimate total infections up to a date.

However, if you take the epi curve and plot the aggregate number of infections you'll find a curve that the Gompertz function approximates quite well.

Then why, pray tell, is the title of the article we are discussing not "Forecasting exponential curves is hard"?

The entire point is that forecasting exponentials is easy but becomes badly wrong at some point and that point is difficult to forecast