An infectious disease like coronavirus, if left unchecked, can spread rapidly through a community. Scientists use the transmission rate, or R0 number, to denote how many people a typical carrier of a disease may go on to infect.
In the absence of control measures such as social distancing or mask wearing, the novel coronavirus has an R0 number of between 2.5 and 3.0, meaning the average carrier would infect between two and three people.
As those people go on to infect others, the disease can spread rapidly through a community that hasn’t been exposed to it before.
However, as more people contract the disease and gain some level of immunity, the disease’s spread will slow naturally.
When enough people become immune such that the whole community is protected, it’s called herd immunity. Herd immunity can sometimes occur naturally from survivors of the disease within a population, but often not without many deaths. Covid-19 has so far killed close to 200,000 people in the U.S. Epidemiologists believe only a small percentage of the nation has been infected and developed some level of immunity.