What it is ain't exactly clear
This is a graph of Covid 19 deaths per million population of the USA and various countries in Europe. It shows that the pandemic has followed a similar curve in these countries, with deaths rising rapidly, and then slowly going down over a period of 60-80 days.
A recent study by an Israeli mathematician proposed that the virus follows a similar curve in most counties, rising up and then going down over a period of 70 days. He doesn't think lockdowns have much effect.
Top Israeli prof claims simple stats show virus plays itself out after 70 days | The Times of Israel
Two recent scientific papers have proposed that their may be some preexisting immunity to covid 19 through previous exposure to other coronaviruses like the common cold.
Using diverse assays for detection of antibodies reactive with the SARS-CoV-2 Spike (S) glycoprotein, we demonstrate the presence of pre-existing immunity in uninfected and unexposed humans to the new coronavirus.
https://www.biorxiv.org/content/10.1...414v1.full.pdf
https://www.cell.com/action/showPdf?...2820%2930610-3