What is a pandemic?

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Pandemics: what are they and can they be stopped? 2:56

(CNN) - The deadly outbreak of the new coronavirus has highlighted the science of disease, raising the question: what exactly is a pandemic?

An outbreak is the occurrence of cases of diseases that exceed what is normally expected, according to the World Health Organization. An epidemic is more than a normal number of cases of illness, specific health-related behavior or other health-related events in a community or region.

However, a pandemic is defined as the "worldwide spread" of a new disease.

The last pandemic reported in the world was the H1N1 flu pandemic in 2009, which killed hundreds of thousands worldwide.

• The coronavirus brings back memories of H1N1, the pandemic that paralyzed Mexico and the United States.

The word "pandemic" comes from the Greek "pandemos", which means all. Demos means population. Bread means all. Therefore, "pandemos" is a concept in which there is a belief that the world's population will likely be exposed to this infection and potentially a proportion of them would get sick, said Dr. Mike Ryan, executive director of WHO's Emergency Health Program of the agency, during a press conference with journalists.

When it comes specifically to the new coronavirus or COVID-19 disease, "what we still don't understand in COVID-19 is the dynamics of absolute transmission," Ryan said.

"We are in a phase of preparation for a possible pandemic," he said. “Let's focus on what we can do and what we have to do, which is to prepare. When we refer to preparing, we mean preparing to detect cases, preparing to deal with cases, preparing to follow contacts, preparing to implement adequate containment measures. ”

'Pandemics mean different things to different people'

Dr. Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, said earlier this month that there is no real scientific and definitive definition of what constitutes a pandemic.

"It really is a semantic limit, to be honest," Fauci said.

He added that there could be arguments on both sides about whether the coronavirus outbreak could be described as a pandemic.

"I think there could be people discussing each end," he said. "Pandemics mean different things to different people."

In general, WHO avoids declaring public health situations that are not influenza pandemics as pandemics. For the flu, the term has been used to describe the preparation for pandemic influenza.

Otherwise, WHO no longer uses an old system that involved describing the phases of pandemic influenza that ranged from the absence of reports of human infections to "a pandemic."

"This change occurred due to the lessons learned from the H1N1 experience in 2009," WHO spokesman Tarik Jasarevic said Tuesday.

For the new coronavirus, “groups of several organizations are working to define the pandemic of this new virus, which could take some time. Meanwhile, our advice remains the same: Member States are strongly recommended to establish plans based on national risk assessments of local circumstances, taking into account the information provided by the WHO global assessments, ”said Jasarevic.

In January, WHO declared that the new outbreak of coronavirus was a public health emergency of international interest.

“Since then, we have seen cases, groups and outbreaks reported in several countries. Most of these cases, groups and outbreaks are traceable, which means that generalized community transmission is not evident. Some countries have even delayed or stopped the transmission. They must remain alert for the possibility of reintroduction. There will probably be more cases in more places, ”Jasarevic said in his email.

"We are at a critical juncture in the outbreak," he said. “While we must continue efforts to contain COVID-19, focusing on strengthening surveillance, conducting thorough investigation of outbreaks to identify contacts and applying appropriate measures to prevent further spread, countries should also take this time to prepare for the possibility of a wider transmission. "

Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention, said on Tuesday: “Ultimately, we hope to see a community spread in this country".

Businesses, schools, communities and families in the United States should prepare for the virus to interrupt their lives, he said. It's not about if, but when and "and how many people in this country will have a serious illness," he said.

“The fact that this virus has caused a disease, including a disease that has caused death, and a sustained spread from person to person is worrisome. These factors meet two of the criteria for a pandemic, ”said Messonnier. "As the spread of the community in more and more countries is detected, the world is about to meet the third criterion: the worldwide spread of the new virus."

Pandemics of the past

The pandemics have been part of the history of mankind for centuries, and one of the first in history dates back to 1580. Since then, at least four influenza pandemics occurred in the nineteenth century and three in the twentieth century, according to Centers for Disease Control and Prevention, the CDC.

The most serious pandemic in recent history was the 1918 flu pandemic, sometimes referred to as the "Spanish flu." It is estimated that the pandemic infected about 500 million people or a third of the world's population and killed about 50 million worldwide.

There is still some debate about where this H1N1 flu virus originated, but scientists have discovered that the virus had genes of avian origin. In other words, he had a connection with the birds.

According to the CDC, more American soldiers died from the 1918 flu pandemic than those who died in battle during World War I in 1918. In 1919, the pandemic declined but the H1N1 virus continued to circulate seasonally for 38 years.

Then, in 1957, a new influenza A H2N2 virus emerged in East Asia, which triggered a pandemic that is estimated to have killed 1.1 million worldwide and 116,000 in the United States. The virus was composed of genes that could be related to an avian influenza A virus, suggesting that it had a connection with the birds.

The virus was first reported in Singapore in February 1957, Hong Kong in April 1957 and in coastal cities in the United States in the summer of the same year. However, its survival in the human population was short and the virus disappeared about a decade after its arrival. Some scientists suggest that it was supplanted by an H3N2 subtype.

In 1968, a pandemic caused by an H3N2 influenza virus that originated in China swept the world. That virus was composed of two genes from an avian influenza A virus, according to the CDC.

The virus was first observed in the United States in September 1968 and caused around 100,000 deaths throughout the country and 1 million worldwide. The majority of excess deaths were in adults over 65, according to the CDC.

The H3N2 virus continues to circulate globally as a seasonal influenza virus.

In the spring of 2009, a new influenza A H1N1 virus emerged. It was first detected in the United States and then quickly spread throughout the world.

The virus contained "a unique combination of influenza genes not previously identified in animals or people," according to the CDC. It was discovered that it was of porcine origin.

During that H1N1 pandemic, the CDC estimated that between 151,700 and 575,400 people died worldwide during the first year the virus circulated. Worldwide, it is estimated that 80% of deaths occurred in people under 65.

• H1N1 flu killed many more people than previously thought

The World Health Organization declared the global H1N1 pandemic in August 2010, but the H1N1 virus continues to circulate as a seasonal flu virus every year.

Now, in early 2020, the world has been waiting to see if the new outbreak of coronavirus could become a pandemic.

"This is unprecedented," wrote Dr. Tom Frieden, former CDC director, in an article published on CNN.com on Tuesday.

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“Apart from influenza, no other respiratory virus has been traced from the emergency to the continuous worldwide spread. The last moderately severe influenza pandemics were in 1957 and 1968; each killed more than one million people worldwide, ”Frieden wrote. "Although we are much more prepared than in the past, we are also much more interconnected, and many more people today have chronic health problems that make viral infections particularly dangerous."

Elizabeth Cohen and Ben Tinker of CNN contributed to this report.

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