CV has a typical spread rate (R0) of >3 (ie each positive case will infect 3 others on average)
Transmission reduction measures (self-isolation when symptoms start, population lockdown, social distancing, etc) and any innate immunity in the population mitigate the actual spread rate.
en.wikipedia.org/wiki/Basic_reproduction_number
Prompt testing means within 48 hours of symptoms developing you know if a person has it, and can then trace and test/isolate contacts who are likely to be about to develop the disease before they are infectious.
This leads to the spread rate falling below 1, and the disease becoming controlled.
South Korea are a prime example of this process.
www.worldometers.info/coronavirus/country/south-korea/
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