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The head of the World Health Organization officially characterized the coronavirus outbreak as a pandemic on Wednesday.

“This is the first pandemic caused by the coronavirus,” the WHO's director-general, Tedros Adhanom Ghebreyesus, said during a news conference.

Until now, the outbreak of the coronavirus that has struck China, Italy, the U.S. and more than 100 other countries and killed more than 4,000 people had been called a public health emergency of international concern.

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The difference centers on just how widespread the virus has become.

"An influenza pandemic occurs when a new influenza virus emerges and spreads around the world, and most people do not have immunity," the WHO states on its website. "Viruses that have caused past pandemics typically originated from animal influenza viruses."

This is the first pandemic caused by a #coronavirus.

We cannot say this loudly enough, or clearly enough, or often enough: all countries can still change the course of this pandemic. This is the first pandemic that can be controlled.https://t.co/dIoa4jYAUN — Tedros Adhanom Ghebreyesus (@DrTedros) March 11, 2020

Tedros said the announcement will not affect how the WHO is confronting the spread of the virus and should not affect how countries are responding.

“Pandemic is not a word to use lightly or carelessly. It is a word that, if misused, can cause unreasonable fear, or unjustified acceptance that the fight is over, leading to unnecessary suffering and death," Tedros said.

“We’re in this together, to do the right things with calm and protect the citizens of the world,” he said. “It’s doable.”

The last pandemic was the 2009 swine flu, caused by the H1N1 virus. That pandemic, which was first detected in Mexico, killed an estimated 200,000 people and hit young adults and children hardest.

The coronavirus pandemic is killing mostly older adults with underlying health conditions. As of Wednesday, there are more than 120,000 confirmed cases of COVID-19 in 114 countries, with 4,373 deaths.