The World Health Organization declared on Wednesday the spread of coronavirus COVID-19 has become a worldwide pandemic after the disease was first detected in China during December and quickly spread to more than 100 locations around the world.

A pandemic is a global outbreak of a serious new illness that requires “sustained transmission throughout the world," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told USA TODAY in February.

In the minds of many, the word "pandemic" is closely connected to the 1918 flu pandemic that killed tens of millions of people, Fauci said.

But by definition, a pandemic doesn't require that scale of destruction. In reality, it's a loosely-defined term.

"81 countries have not reported any #COVID19 cases, and 57 countries have reported 10 cases or less. We cannot say this loudly enough, or clearly enough, or often enough: all countries can still change the course of this pandemic," reads a statement attributed to Tedros Adhanom Ghebreyesus, the director-general of the WHO.

Here's what you need to know about why the spread of coronavirus COVID-19 has been declared a pandemic:

Why did it take so long for coronavirus to be called a pandemic?

The WHO cited inaction as a major reason for making the declaration at this time.

"WHO has been assessing this outbreak around the clock and we are deeply concerned both by the alarming levels of spread and severity, and by the alarming levels of inaction," reads a statement attributed to Tedros.

"We have therefore made the assessment that #COVID19 can be characterized as a pandemic."

The World Health Organization previously defined a pandemic as "the worldwide spread of a new disease." But there is no strict definition for how serious the illness should be, and previous pandemics have varying fatality rates.

A disease spreading around the world may not be as unusual as it sounds, according to William Schaffner, an infectious disease specialist at Vanderbilt University.

Advances in technology have made it easier to detect and track new diseases, he told USA TODAY in February. It's possible that some diseases previously circled the globe undetected, he said.

Is a pandemic different than an epidemic?

Yes. While an epidemic describes an illness affecting a defined region, a pandemic has a global impact.

In February, Fauci explained why coronavirus hadn't yet met the definition of a pandemic. At that time the virus' spread in other countries has not yet been sustained for a significant amount of time. And since many of the cases outside China were related to travel, the virus' global impact wasn't yet considered widespread.

That situation has evolved in recent weeks. Currently in the U.S., COVID-19 cases include people who were traveling to an impacted region, close contacts of a known case as well as infections acquired in the community with an unknown source, the CDC says.

Should you worry about the coronavirus pandemic?

Take typical flu-season precautions: Avoid close contact with people who are sick. Don't touch your eyes, nose and mouth. Cover your cough. Stay home when sick. Clean household objects and surfaces. Wash your hands for at least 20 seconds.

If you're sick and want to prepare for a possible quarantine, here's a shopping list of items you should consider buying.

And no, you don't need a face mask unless you have COVID-19 and are showing symptoms. Buying up masks takes away precious materials from the health workers who need them most.

Contributing: Ken Alltucker, Jayne O'Donnell, David Jackson and Grace Hauck