There is an interesting US article here about the exponential growth associated with covid 19. Studies seem to assume a doubling of cases every 6 days which will equate to 1 million US cases by the end of april
www.statnews.com/2020/03/10/simple-math-alarming-answers-covid-19/.
There is more about exponential growth here
ourworldindata.org/coronavirus
Based on the global WHO data up to and including 9th March 2020, the doubling time for COVID-19 is as follows:
Doubling time for the global number of cases (including China): 19 days
Doubling time for the global number of cases (excluding China): 5 days
The fact that the doubling time is longer when China is included is due to the fact that the number of daily cases has declined after the lockdown in China.
As we said previously, the doubling time will change and it would be wrong to make projections based on a constant doubling time. But it is important to remind ourselves of the nature of exponential growth.
If during an outbreak the number of cases is in fact doubling and this doubling time stays constant, then the outbreak is spreading exponentially.
Under exponential growth 500 cases grow to more than 1 million cases after 11 doubling times.7 And after 10 more doubling times it would be 1 billion cases.
This is in no way a prediction for the number of cases we need to expect; it is simply a reminder that exponential growth leads to very large numbers very quickly even when starting from a low base.
And it is important to be reminded of the nature of exponential growth because most of us do not grasp exponential growth intuitively. Psychologists find that humans tend to think in linear growth processes (1, 2, 3, 4) even when this is not appropriately describing the reality in front of our eyes. This bias – to “linearize exponential functions when assessing them intuitively” – is referred to as ‘exponential growth bias’.
Psychological research shows that “neither special instructions about the nature of exponential growth nor daily experience with growth processes” improved the failure to grasp exponential growth processes.