But an outbreak of flu that began as the war ended, turned out to be far more destructive.

It was known as the Spanish Influenza, and infected one in three humans on earth.

It was the worst pandemic in modern history and claimed the lives of between 50 and 100 million people.

Now, as a new strain of coronavirus spreads across the world, when does a disease become a pandemic and if it does, what happens next?

Disease experts use the term "pandemic" when a new infection spreads to multiple countries and continents at the same time, affecting many people.

That's different from another term, epidemic, which describes an infection outbreak that's larger than usual, but stays confined to a single location or region.

So when does a disease officially become a pandemic?

The World Health Organization is the body that decides when an infectious disease formally becomes a pandemic, but that choice is not always black and white.

The group's director-general says the WHO assesses whether to use the word "pandemic" by evaluating three things.

First is the geographical spread of the virus.

Second is the severity of the disease caused by the virus and lastly, the societal impact of the disease.

A disease is more likely to become a pandemic if it is caused by a new strain of a virus, as is the case with Coronavirus Disease 2019 or COVID-19.

The ease with which it infects people and spreads from person to person also play a role in the designation.

Past pandemic outbreaks have typically originated from animal viruses, before crossing over to humans.

These can spread rapidly around the world because people do not have the immunity needed to fight the new infection.

COVID-19 was declared a pandemic in March 2020, the sixth pandemic declared in about a century.

The 1918 Spanish flu was easily the deadliest flu pandemic of the 20th century, killing tens of millions of people.

The Asian flu outbreak followed in 1957, killing roughly 1.1 million people around the world.

Thankfully scientists were able to develop a vaccine quickly, effectively containing its spread.

Another influenza outbreak, the Hong Kong flu, started to spread from China in 1968.

It was caused by a compound virus, which combined the Asian virus from ten years earlier with a form of bird flu.

It killed around one million people—most of them older than 65.

HIV, which was first identified as the virus behind AIDS in 1983, was also considered a pandemic.

The human immunodeficiency virus severely damages the cells in your immune system and weakens your ability to fight everyday infections and disease.

In the past 40 years, it's killed 35 million people worldwide, about half of the people who were infected by the virus.

Then in 2009, a new outbreak, initially called the swine flu, was named a pandemic.

It infected nearly 61 million people, and experts estimate it killed up to 575,000 people in a single year.

The WHO declared the pandemic over in August 2010, but the virus has continued to circulate as a seasonal flu ever since.

In recent years, the WHO has changed how it decides whether an outbreak constitutes a pandemic, following criticism that the threat of the 2009 swine flu had been exaggerated.

Many governments stockpiled vaccines which ultimately went unused, while pharmaceutical companies profiteered from the ensuing panic.

The disease turned out to be milder than was originally thought.

Since then, the WHO has released a guide to manage flu pandemics at a national and international level.

According to its pandemic preparedness plan, national governments are required to follow specific protocols – if a pandemic is declared – to prevent or reduce the spread of a virus.

For instance, authorities at a regional and local level must fully mobilize health systems, hospitals and medical workers.

In addition, healthcare providers must plan for a surge in patients, and offer protective equipment to their workforce.

Governments must also limit social interaction, initiate quarantine measures and enforce isolation procedures.

Upgrading a disease to a pandemic outbreak also has psychological implications for how we think about a disaster.

According to the WHO "using the word pandemic carelessly has no tangible benefit, but it does have significant risk in terms of amplifying unnecessary and unjustified fear and stigma."

Six months before the latest coronavirus outbreak, a WHO report noted that "many countries still lack a national pandemic influenza preparedness plan."

So what are the economic costs of a pandemic?

A previous coronavirus strain called SARS, which was detected in 2002, wasn't widespread enough to become a pandemic.

While it only infected more than 8,000 people, it still cost the global economy more than 50 billion USD in 2003.

You see, advances in medicine, communication and technology have brought mortality rates down.

But greater trade flows and cheaper air travel have seen the world economy become ever more interconnected and that causes the costs of a pandemic to rise.

A report now estimates that a pandemic will cost 570 billion USD a year.

That represents 0.7 percent of the world's total income.

A pandemic can overwhelm global health systems.

It can also force infected individuals to avoid the office or work less productively.

The fear of infection spread forces people to stay apart.

And that can be even more debilitating… shutting down schools, businesses and public services.

Insurance companies must also watch developments closely.

A pandemic can mean more travel claims, more hospital claims and choke up global supply chains.

The impact on corporate earnings can then cascade into financial markets all around the world.

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