A pandemic is an outbreak of infectious disease that spreads across multiple countries or continents, affecting large numbers of people. The World Health Organization defines it simply as “a new disease with worldwide spread.” What separates a pandemic from a regular outbreak is geographic scale: the same disease causing illness in one city is an outbreak, spreading rapidly through a country or region is an epidemic, and crossing into multiple world regions with sustained transmission is a pandemic.
What Makes a Disease Become a Pandemic
Three conditions need to line up for a pandemic to take hold. First, a pathogen emerges that humans have little or no existing immunity to. Second, that pathogen develops the ability to spread continuously from person to person, not just from animals to people in isolated cases. Third, that transmission sustains itself across communities in multiple regions of the world.
The vast majority of pandemic threats start in animals. Between 60 and 75 percent of all emerging human infectious diseases originate in non-human animal species. A virus circulating in birds, pigs, or bats undergoes genetic changes that allow it to infect humans, and then further changes that let it pass efficiently between people. This jump from animals to humans is called zoonotic spillover, and it’s the starting point of nearly every major pandemic in history.
Not every new disease becomes a pandemic. Many zoonotic viruses cause isolated infections or small clusters without ever achieving sustained human-to-human transmission. The difference often comes down to how the virus spreads (respiratory droplets travel far more efficiently than bodily fluids, for example) and how long people are contagious before they feel sick.
How Pandemics Are Officially Declared
The WHO uses a phased system, originally designed for influenza, to track how close a new disease is to pandemic status. In the early phases, a new virus causes sporadic human cases but doesn’t spread efficiently between people. Phase 4 marks the first verified human-to-human transmission capable of causing community-level outbreaks. By Phase 5, the virus has spread between people in at least two countries within one WHO region. Phase 6, the pandemic phase, is declared when community-level outbreaks are confirmed in at least two different WHO regions, signaling that global spread is underway.
In 2024, WHO member states adopted amendments to the International Health Regulations that introduced a new alert level called a “pandemic emergency.” This sits above the existing “public health emergency of international concern” designation and is meant to trigger stronger international coordination when a health threat escalates to pandemic scale. The amendments also include provisions to improve access to medical supplies and financing across countries, with a focus on equity. Eleven of 196 member states rejected the amendments.
Pandemic vs. Epidemic vs. Endemic
These three terms describe different stages in how a disease behaves in a population. An epidemic is the rapid spread of a pathogen within a population, typically within a country or region. A pandemic is an epidemic that has crossed international and continental boundaries. Endemic describes the phase that often follows: the disease settles into a stable, predictable pattern at lower levels, maintained by a balance between new infections and population immunity built up through prior illness or vaccination.
A disease becomes endemic when the average number of people each infected person passes the virus to holds steady at around one. At that point, cases still occur, but they no longer surge unpredictably. Seasonal flu is a classic example of an endemic disease. COVID-19 has largely transitioned into this pattern in most parts of the world, though that transition was gradual rather than marked by a single declaration.
Notable Pandemics in History
The 1918 influenza pandemic remains the most frequently referenced modern pandemic. For decades, the death toll was commonly cited as 50 to 100 million people worldwide. More recent analysis, however, suggests the true number was likely closer to 17 million deaths across 1918 and 1919 combined, and that the widely cited figure of 100 million is very unlikely based on the demographic data available. Even at the lower estimate, the 1918 pandemic killed at an extraordinary rate, roughly 788 per 100,000 people in its peak year.
COVID-19, declared a pandemic in March 2020, had a confirmed death toll that significantly undercounts the true impact. The WHO estimated 14.9 million excess deaths globally in 2020 and 2021 alone, while the Institute for Health Metrics and Evaluation put the figure at 18.2 million for the same period. “Excess deaths” captures not just confirmed COVID cases but also deaths from overwhelmed health systems and disrupted care for other conditions.
Other recognized pandemics include the 1957 Asian flu, the 1968 Hong Kong flu, the 2009 H1N1 swine flu, and the ongoing HIV/AIDS pandemic, which has killed an estimated 40 million people since the early 1980s. Each varied enormously in severity, speed of spread, and the populations hit hardest.
Economic and Social Disruption
Pandemics cause damage far beyond direct illness. COVID-19 triggered the sharpest global economic downturn since the Great Depression. Global GDP fell 3.0 percent in 2020 after growing 2.8 percent in 2019. That reversal translates to trillions of dollars in lost economic output, but the effects were uneven: countries dependent on tourism, service industries, and informal labor were hit hardest, while some technology sectors grew.
The social costs are harder to quantify but equally significant. School closures affected over a billion children worldwide during COVID-19. Mental health burdens increased sharply. Supply chain disruptions caused shortages of goods ranging from medical equipment to consumer products. These cascading effects are part of why pandemic preparedness receives so much attention from governments and international organizations, even during years when no pandemic is active.
Global Preparedness Today
Despite the lessons of COVID-19, most countries remain poorly prepared for the next pandemic. The Global Health Security Index, which scores countries on their ability to detect, prevent, and respond to infectious disease threats, found that the average country scored just 38.9 out of 100 in its most recent assessment. That average was essentially unchanged from two years earlier, suggesting that COVID-19 did not produce meaningful improvements in most nations’ readiness.
Even top-ranked countries scored well below a perfect mark. The United States led at 75.9, followed by Australia at 71.1, Finland at 70.9, and Canada at 69.8. The gap between the best-prepared and least-prepared nations is enormous, and since pandemics by definition cross borders, the weakest links in the global system shape outcomes for everyone. Early detection, rapid information sharing, and equitable access to vaccines and treatments remain the core pillars that public health systems are working to strengthen.