pandemic pandemic a pandemic (/pænˈdɛmɪk/ pan-dem-ik) is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. widespread endemic diseases with a stable number of infected individuals such as recurrences of seasonal influenza are generally excluded as they occur simultaneously in large regions of the globe rather than being spread worldwide. early in the covid-19 pandemic, convention centers (pictured here) were deemed to be ideal sites for temporary hospitals, due to their existing infrastructure (electrical, water, sewage). hotels and dormitories were also considered appropriate because they can use negative pressure technology. throughout human history, there have been a number of pandemics of diseases such as smallpox. the most fatal pandemic in recorded history was the black death—also known as the plague—which killed an estimated 75–200 million people in the 14th century. the term had not been used then but was used for later epidemics, including the 1918 influenza pandemic—more commonly known as the spanish flu. current pandemics include hiv/aids and covid-19. definition depiction of the burial of bodies during black death, which killed up to half of eurasia's population in the 14th century.american red cross workers carry a body during the 1918–20 "spanish flu" pandemic. a pandemic is an epidemic occurring on a scale that crosses international boundaries, usually affecting people on a worldwide scale. a disease or condition is not a pandemic merely because it is widespread or kills many people; it must also be infectious. for instance, cancer is responsible for many deaths but is not considered a pandemic because the disease is not contagious—i.e. easily transmittable—and not even simply infectious. in 2009, the world health organization (who) dropped the words "with enormous numbers of deaths and illness" from their definition. in 2008, it also dropped the requirement of an "influenza pandemic" to be a new sub-type with a simple reassortant virus, meaning that many seasonal flu viruses now could be classified as pandemic influenza. the word comes from the greek παν- pan- meaning "all", or "every" and δῆμος demos "people". assessment stages for a novel influenza virus, the world health organization previously applied a six-stage classification to describe the process by which the virus moves from the first few infections in humans through to a pandemic. it starts when mostly animals are infected with a virus and a few cases where animals infect people, then moves to the stage where the virus begins to be transmitted directly between people and ends with the stage when infections in humans from the virus have spread worldwide. in february 2020, a who spokesperson clarified that "there is no official category ". 2009 world health organization (who) influenza pandemic phase descriptions phase probability transmission route 1 uncertain animal-to-animal infection only – 2 uncertain animal-to-human infection (considered a human pandemic threat) 3 uncertain sporadic or clustered cases in humans no sustained community-level outbreaks 4 medium to high human-to-human transmission sustained community-level outbreaks 5 high to certain sustained in two countries in one who region 6 pandemic in progress sustained in-country in another who region post-peak levels drop below the peak in most countries possible new wave activity rising again in most countries post-pandemic levels return to ordinary seasonal levels among countries with adequate surveillance during the 2009 influenza pandemic, keiji fukuda, assistant director-general ad interim for health security and environment, who said "an easy way to think about pandemic ... is to say: a pandemic is a global outbreak. then you might ask yourself: 'what is a global outbreak?' global outbreak means that we see both the spread of the agent ... and then we see disease activities in addition to the spread of the virus." in planning for a possible influenza pandemic, the who published a document on pandemic preparedness guidance in 1999, which was revised in 2005 and 2009. it defined phases and appropriate actions for each phase in an aide-mémoire titled who pandemic phase descriptions and main actions by phase. the 2009 revision, including descriptions of a pandemic and the phases leading to its declaration, was finalized in february 2009. the 2009 h1n1 virus pandemic was neither on the horizon at that time nor mentioned in the document. all versions of this document refer to influenza. the phases are defined by the spread of the disease; virulence and mortality are not mentioned in the current who definition, although these factors have previously been included. influenza intervals in the cdc's pandemic intervals framework in 2014, the united states centers for disease control and prevention introduced an analogous framework to the who's pandemic stages titled the pandemic intervals framework. it includes two pre-pandemic intervals, investigation recognition and four pandemic intervals, initiation acceleration deceleration preparation it also includes a table defining the intervals and mapping them to the who pandemic stages. severity