Exponential spread, also known as exponential growth, is something you might remember from math class. It's the concept that the bigger something gets in numbers or size, the faster it begins to grow.
Think of it like a snowball rolling downhill: The bigger a snowball gets, the faster it rolls (thanks, gravity), which makes it collect more snow and grow bigger and bigger.
When it comes to coronavirus, this concept is essential because it helps us understand how damaging and fast-spreading the virus can be if we don't take precautions to slow the rate of infection. Because COVID-19 is so contagious, the number of infected people will grow exponentially if left unchecked.
For example, as soon as we hit 100 cases in the U.S. (March 2), it took less than one week for that number to increase five times and hit 500 (March 8). Three days later (March 11), that number was at 1,000. Five days later (March 15), we had 3,000 confirmed cases. By March 17, it was 5,000. March 19, we were at 9,400 cases.
But the exponential spread of COVID-19 depends on three things: 1. At least one infected person in a population. 2. Large numbers of uninfected people in the community. 3. Regular contact between infected and uninfected people.
Plus, there's a segment of the population—possibly one out of every four people—who don't have any symptoms but can still pass on the virus.
That is how, in the real world, the exponential spread makes something like COVID-19 spread very, very quickly. One person gives it to five people; those five people give it to five other people each; now twenty-five other people are giving it to … on and on.
The one part of the equation that is sort of hopeful is that as more people become infected, there will eventually be fewer people who are catching it. This, combined with people who are quarantined and stop having contact with others, allows the numbers to start to taper off. On March 19, after 11 weeks of exponential spread, China reported no new local infections for the first time. Unfortunately, we are not there yet. And it's not clear when we will be.