This so-called exponential curve has experts worried. If the number of cases were to continue to double every three days, there would be about a hundred million cases in the United States by May.

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That is math, not prophecy. The spread can be slowed, public health professionals say, if people practice “social distancing” by avoiding public spaces and generally limiting their movement.

Still, without any measures to slow it down, covid-19 will continue to spread exponentially for months. To understand why, it is instructive to simulate the spread of a fake disease through a population.

We will call our fake disease simulitis. It spreads even more easily than covid-19: whenever a healthy person comes into contact with a sick person, the healthy person becomes sick, too.