The last disease the WHO declared a pandemic was a new flu strain, initially called "swine flu," in 2009. That decision came after the new H1N1 flu had been spreading in multiple countries for about six weeks. Today, that strain is what's known as "endemic" worldwide it became part of every season's regular flu outbreak.

This is the first time this kind of virus a coronavirus has been labeled a pandemic, "but at the same time, we believe that it will be the first also to be able to be contained or controlled," Tedros said.

OUTBREAK? EPIDEMIC? PANDEMIC? WHAT'S THE DIFFERENCE?

An outbreak is a sudden rise in cases of a disease in a particular place. An epidemic is a large outbreak. A pandemic means a global epidemic.

Experts point out that the word "pandemic" is sure to reverberate, even though many people may not understand what it means.

"The word hasn't been suitably explained and made clear," said Ian Mackay, who studies viruses at Australia's University of Queensland. "It has been kept in a drawer and only used at the worst time. So, of course people have fear of it."

WHAT DOES THIS MEAN ABOUT THE DISEASE'S SEVERITY?

Pandemic is a scary word but it has nothing to do with how serious the illness is. It just means a disease is spreading widely. There can be pandemics of mild illness, like that H1N1 flu turned out to be in 2009.

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Q&A: What the pandemic declaration means | Health and Fitness - Lincoln Journal Star