It seems like the intro of this page is pretty good for highlighting past diseases that had that status and a general sentence on how a virus qualifies for that status.
Between 2009 and 2022, there were seven PHEIC declarations: the 2009 H1N1 (or swine flu) pandemic, the 2014 polio declaration, the 2013–2016 outbreak of Ebola in Western Africa, the 2015–16 Zika virus epidemic,[5] the 2018–20 Kivu Ebola epidemic,[6] the ongoing COVID-19 pandemic,[7] and the ongoing 2022 monkeypox outbreak.[
A public health emergency of international concern (PHEIC) is a formal declaration by the World Health Organization (WHO) of "an extraordinary event which is determined to constitute a public health risk to other States through the international spread of disease and to potentially require a coordinated international response", formulated when a situation arises that is "serious, sudden, unusual, or unexpected", which "carries implications for public health beyond the affected state's national border" and "may require immediate international action"
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https://en.wikipedia.org/wiki/Public_health_emergency_of_international_concern
It seems like the intro of this page is pretty good for highlighting past diseases that had that status and a general sentence on how a virus qualifies for that status.