In the past two weeks, the number of cases of COVID-19 outside China has increased 13-fold.

COVID-19 can be characterized as a pandemic.

The coronavirus, or COVID-19 disease, had already overwhelmed China, South Korea, Iran, and Italy.

And this was a warning to other countries where it was now spreading quickly.

In the days and weeks ahead, we expect to see the number of cases, the number of deaths, and the number affected countries climb even higher.

The spread of COVID-19 was no longer something that could be stopped.

But we can still slow it down.

We just have to act right now.

Someone with COVID-19 usually develops a fever and a cough.

Aches, pains, and other mild symptoms are also possible, but are less frequent.

But the severity of those symptoms varies, and for some people who get the virus, you might never show symptoms at all.

Based on the data from China, the vast majority of cases are not life-threatening.

In 80 percent of cases, people experience only mild disease.

But in 20 percent of cases, the disease can manifest in a more serious way.

It can develop into quite a severe pneumonia, where people need to be hospitalized and put on ventilators.

Overall, it seems like about 1 to 2 percent of known cases lead to death, but that rate is much lower for young people, and much higher for the elderly.

And it also seems as if people with unmanaged underlying chronic diseases, they also have a tougher time overcoming the virus.

The virus also seems to be very contagious, more contagious than the flu.

All you need to do to spread COVID-19 is cough or sneeze on someone else.

Touch a surface where the virus still lives, and then put your hand in your mouth or your eyes or your nose.

After getting infected, it can take an average of five to six days before you feel sick, and your symptoms start to appear.

But you can already spread it to other people in that period, even if you feel healthy.

Just as people realize they're sick, they seem to be at the most risk of passing it along to others.

That's how the virus has been so effective at spreading across the world so quickly and why the WHO was now calling COVID-19 a pandemic.

But what they said next was just as important.

We cannot say this loudly enough.

All countries can still change the course of this pandemic.

And that depends on something each of us needs to do as individuals.

So diseases become really dangerous when everyone gets sick at once, and the health system becomes overwhelmed.

In any hospital, the capacity to treat patients is limited by how many beds they have.

Think of this as the number of beds in your local hospital at any given time.

A couple are already filled by patients receiving treatment for things like a car accident injury or a stroke.

And this dot represents one person who's healthy and decides to go out like usual.

They jump on the subway and head into the office, where they catch COVID-19.

But they don't feel sick right away, and might not for several days.

So, later they go to a basketball game, where they unknowingly infect two or three more people.

Most of these people will have relatively mild cases, but one might be an elderly person with a severe case who will eventually have to go to the hospital.

But these three, who are all infected but don't feel sick, go out again.

On the subway, into the office, and then out after work, infecting several more people, twenty percent of whom will need to go to the hospital.

Over a short period of time, this process multiplies the number of people going to the hospital each day.

Before long, the hospital is full and a crisis begins.

People with severe cases of COVID-19 can't get treatment, and some who could be saved, die.

Plus, people with other issues can't get treatment either and some of them die.

This surge of severe cases causes avoidable deaths.

That's what happened in South Korea, Iran, and Italy, all of which went from 100 to more than 5,000 cases in less than two weeks.

A lot of people died because they couldn't get into the hospitals.

This surge is made up of only the severe cases, but it was generated by people who didn't feel sick spreading the disease in public.

Which means the people who can do the most to avoid these unnecessary deaths, are these people.

And that means all of us.

To slow the virus down, you need to act as if you already have it.

By avoiding public transportation, the office, crowded places, and even small social gatherings, you decrease your chances of both getting the disease, and spreading it.

This is called social distancing.

If enough of us do it, the virus still spreads—but much slower.

Over time, many people might still get infected, but fewer severe cases show up to the hospital each day, never overwhelming the system.

This trendline gets flatter, these people can all get treatment, and fewer people die because of it.

These are the two ways the COVID-19 pandemic can play out.

But this one only happens if everyone does their part.

And it's why experts and officials are urging people to "flatten the curve" by social distancing, and staying home as much as possible.

It's also why In the U.S., many companies are helping by requiring employees to work from home and major sports leagues have canceled their games for the time being.

It may seem drastic but it's worked before.

In 1918, the cities of Philadelphia and St. Louis were both hit by a flu pandemic, but they responded in different ways.

In Philadelphia, health officials allowed a huge parade to go ahead.

While in St. Louis, officials prepared.

They closed schools, theaters, and bars.

Philadelphia's hospitals were overwhelmed and many more died as a result.

But St. Louis was able to avoid those excessive deaths.

A hundred years later, these are the two scenarios we face.

A difference not in whether you get the coronavirus, but when you get it.

That could mean the difference between life and death, maybe for someone you know.

We have to act now.