The problem with using a growth calculator to predict infectious disease spread is that you're missing way too many variables:
1) People are creatures of habit and congregate in familiar places. If you're infected and your routine is just home-work-home, with the occasional stop at a fast food drive-thru window, grocery, gas station pump thrown in, the possibility of you, a single person, directly infecting others eventually tails off. This isn't like compounding interest on your dividend returns or a savings account.
2) People the index patient infects then go off into their own spheres of influence and infect new people, but with similar diminishing returns.
3) People cannot be infected again while they are already themselves infected carriers. See 1) You have to remove the compounding effect of infected carriers. If the index patient starts the infection in 2.6 people of different groups (the R-naught of COVID-19), there stands to reason those groups have overlap of common interest with the index patient where they cannot infect each other in increasing multiples because they're now all already carriers. Multiples in the same sphere cannot infect each other exponentially (E.g.; A church congregation cannot all produce an exponentially linear multiplicative infection rate when their interactions are largely with each other).
4) Rates of incubation and decay. Are previously infected and recovered carriers now immune? (We don't know. Early returns from Wuhan are this is persistent like the flu. Recovery does not convey lifelong immunity).
5) Remove the deceased from the numbers.
So, when calculating infectious disease spread, you have to consider three different equations (The SIR model) using Euler's Method.
http://mat.uab.cat/matmat/PDFv2013/v2013n03.pdf
In layman's terms, you have a multiplicative rate of growth from the index patient to 2.6 people. That doesn't happen daily; it may take 7-14 days. Then you have another compounding from those people that grows exponentially from there in waves. Doubling every six days. Then doubling every five days. Every three days. Then doubling daily, until it cannot double anymore (while still accounting for the fact that maybe 30% of the global population will never come in contact with the infected 70%).
The spread rate eventually goes vertical and 7 or 8% of the infected die because you can only save
x number of people a day. (Italy)
This doesn't happen in a vacuum. "Flu season" is present
right now. >50% of this nations' respirators are already occupied by people with pneumonia and emphysema complications induced by the flu.
