Coronavirus is spreading so quickly that our brains can't keep up. Experts explain why.

The number of coronavirus deaths in the United States rose from 1 to 100 in a little more than two weeks. About a week later, the number skyrocketed to 1,000.

Those numbers are tragic. For many people, they're also disorienting and confusing. How did the number get so high so fast?

And while spikes in number of confirmed cases can be in part attributed to better testing, the number of deaths also shows a disease that is spreading alarmingly fast.

You're right to be baffled by the rapid spread of the virus, according to Columbia University's Hod Lipson. The sudden spike is an example of a concept that is "fundamentally difficult ... for the human mind to understand."

You might remember it from middle or high school: exponential growth.

The term is thrown around in our daily vocabulary, but in the case of coronavirus, it has a technical meaning that will help you understand why the virus is spreading so quickly — and how its course could be altered.

The doubling penny

Lipson used a common example to explain how exponential growth tricks your mind.

Pick which option you think will make you the most money in a month:

• $10,000 on every day of the month.
• A penny that doubles in value every day.

The correct answer is the penny.

The first option will leave you with about $300,000 in a month. The magic penny will be worth several million dollars.

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That's an example of exponential growth, and it's something we don't see very often in real life, said Lipson, who often talks about exponential growth as it relates to computing power and artificial intelligence.

At first, the process is "deceptively slow," he said. But as the numbers get bigger, they grow so rapidly it's hard for our minds to keep up.

In our example, a cent becomes two in a single day at the beginning of the month. But by the end, 1  million would become 2 million in a day. 

In our day-to-day experience, growth usually happens at a much steadier rate — more like the $10,000 option.

What does all this have to do with coronavirus?

A highly contagious virus like the one that causes COVID-19 has the potential to spread exponentially — and in many areas of the world, it is currently doing so.

That's because of its reproduction number — how many healthy people a sick person infects on average.

For COVID-19, that number is likely higher than two, according to Melissa Nolan, an infectious disease expert and professor at the University of South Carolina.

One sick person quickly becomes two; two becomes four; four becomes eight. Graph it on a chart, and the line quickly skyrockets upward.

It's worth noting that our ability to test for the virus makes all this data messy, Nolan said. In some cases, spikes in confirmed cases says more about our ability to test for the virus than it says about how fast the virus is actually spreading.

Will coronavirus cases keep growing exponentially?

Not forever. The question is when the growth will slow and why.

This is the idea behind "flattening the curve": The virus' exponential spread can be slowed down.

Nolan gave a few examples of how that might play out:

• Social distancing: Keeping people apart will make it harder for the virus to spread. But exactly what impact it has is difficult to predict and will likely vary by location.
• The virus itself: Many viruses mutate and adapt. This could happen in the case of COVID-19, but it's impossible to say at this time what effect a mutated virus would have.
• Herd immunity: Many experts believe people who recover from the virus will be immune from it for a time, although more research is needed on the subject. As more people become immune, the harder it is for the virus to spread. This is also the concept behind how a vaccine can halt the spread of a disease.

The math lesson you probably forgot

If you want to revisit the math behind exponential growth, here's a quick refresher.

You may remember expressions that look like this: 2⁴

That means "two, multiplied by itself four times". That kind of calculation forms the basis of exponential growth.

Michelle Boyd — a high school math teacher in York County, Pennsylvania — said students learn the foundations of the concept in middle school.

They'll apply those lessons later in financial calculations, statistics, algebra and trigonometry. For some of her students, she brings the lesson home by doing hands-on activities demonstrating exponential growth — the results are often shocking to them.

Exponential growth is so difficult to visualize that it is often graphed in a special way, Lipson said.

It's called a logarithmic scale, and it's easy to spot: Just look for graphs, like the one below, that have huge jumps in their scale. 

Notice that the second tick mark after 100 is not 200 — it's 1,000. That big jump in scale flattens out the exponential data.

This makes for a much neater presentation and can help the viewer spot trends, Lipson said.

But it also is "deceptively disarming," he said.

If you used a regular scale on the chart, the lines in the graph above would all essentially point straight up.