https://en.wikipedia.org/wiki/Basic_reproduction_number
Or, in a population "available" to the virus, a population with a certain social behaviour, that "k" will initially be equal to 2.5 of 3 (= R0 of CoVid-19), so we get exponential divergation.
The more people get infected, sick, immune or die, the less the POTENTIAL infected people also become sick. If a fraction X of people become immune, then, of that potential-infected 2.5 or 3 that would be infected, only 1-X amount "available" to become sick.
So k becomes R0 * (1 - X)
When does the epidemy stop: if k becomes 1. At first, k = R0, and when more and more first become sick to then die or become immune, k drops because X rises.
So not everybody has to become sick. It suffices that k becomes 1.
That's when R0 * (1 - X) = 1, or put the other way: when X equals 1 - 1/R0
So as long as not 60% (R0 of CoVid-19 is estimated as 2.5-3%) of the population became immune, you cannot remove your restrictions.
It's the same as in a nuclear power reactor, where they stopped the fission with a confinement borium rod. You can't remove the bar without restarting the fission. Unless the very last neutron disappeared, which is impossible, and there are also neutrons from cosmic origin. You can never remove the confinement bar.
So there is only one exception case: if the very last covid-19 patient in the entire world died or became immune. And much earlier than then, another virus family/type can have popped up, that the human immune system again doesn't recognize, starting over the entire story.
De Spanish flu killed 50 - 100 miljoen people in 2 or 3 months in 1918, to then disappear nearly completely.
Came back in 1919, to make sick the people that "protected" themselves in 1918. In the end, the estimation was a 25-70% of worlds population had have gotten it. They mournt their deaths, they stopped shedding tears to then continu life.
What governments now did / do in 2020, is purposely halting peoples life, by forced upon suicidal restrictions.
Let's hope for the best for everybody. I leave a positive snippet from an ICU Doctor worth following. 
