Just over 100 years ago, a new strain of influenza infected a third of the world’s population — but within just three years, the threat of this deadly flu had all but passed.

This was a time before modern medical care and even before humans understood what viruses were. So what’s changed since then?

It’s a question plenty of you have asked in recent months: how do pandemics end? And how will the one we’re currently living through end?

Three ingredients for a pandemic

An infectious agent needs three conditions to cause a pandemic, says virologist Kirsty Short from the University of Queensland:

1. 1.It needs to cause disease in humans
2. 2.It needs to be highly transmissible
3. 3.We need to have no pre-existing immunity to it

“For example, we live with MERS today,” says Dr Short, referring to Middle Eastern respiratory syndrome, a coronavirus that is related to the one that causes COVID-19.

“It hasn’t caused a pandemic yet because it’s not highly transmissible from person to person.

“In contrast, the seasonal coronaviruses that we get, probably at one point were a pandemic, and they’ve just become these seasonal colds that we don’t really care about because we’ve evolved immunity to them.”

In terms of the three ingredients that make a pandemic, when it comes to COVID-19 there’s not much we can do to stop the coronavirus from infecting us, because that’s based on the biology of the virus and us as humans.

With physical distancing and masks, we are somewhat able to pull that second lever of not allowing the virus to transmit as much.

But the big thing that stops a virus becoming a pandemic — that is, a large-scale outbreak affecting multiple countries or continents — is the third factor: immunity.

“Herd immunity can be achieved through natural infection or vaccination,” Dr Short says.