Influenza A pandemic can occur when a type of influenza virus, known as the influenza A virus, mutates suddenlyTrusted Source.

This change can result in what the body sees as a completely new virus. The major and abrupt change from a recognizable virus to a new one is called an antigenic shift.

On the surface of the virus are HA proteins and NA proteinsTrusted Source. If one or both of these change, a new influenza A virus subtype can result. Influenza viruses have an H figure and an N figure. Swine flu, for example, is also known as H1N1, while avian flu has the subtype H5N1.

If a flu subtype gains the ability to spread rapidly between people, a pandemic may result.

After the pandemic emerges and spreads, humans develop some immunity over time. The virus subtype may then circulate among humans for several years, causing occasional flu epidemics.

Various organizations around the world, such as the WHO and the Centers for Disease Control and Prevention (CDC), monitor the behavior and movement of influenza viruses.

Their findings help health authorities develop strategies for controlling the spread and impact of influenza.


The Spanish flu pandemic, from 1918 to 1920, claimed 100 million lives. Experts consider it the most severe pandemic in history. The Black Death was fatal for more than 75 million people in the 14th century.

Some pandemics that have occurred throughout history include:

  • 541–542: Plague of Justinian
  • 1346–1350: The Black Death
  • 1899–1923: Sixth cholera pandemic
  • 1918–1920: Spanish flu (H1N1)
  • 1957–1958: Asian flu (H2N2)
  • 1968–1969: Hong Kong flu
  • 2009–2010: Swine flu (H1N1)
  • 2020: COVID-19

Animals carry some viruses that rarely spread to humans. Sometimes, these viruses can mutate and become transmissible to and between people.

When an animal virus first passes to humans, health authorities focus on it as a potential pandemic. This transmission indicates that a virus is mutating and might become highly contagious and harmful.

Swine flu and avian flu are viral diseases that were common in pigs and birds, respectively, but not humans. This changed once an antigenic shift occurred.

In recent years, there has also been concern about viruses that experts have linked to camels (Middle East Respiratory Syndrome, or MERS-CoV) and monkeys (Ebola).


The WHO have a six-phaseTrusted Source program for identifying potential flu pandemics:

  • Phase 1: No local health authorities have reported that an influenza virus circulating among animals can cause disease in humans.
  • Phase 2: An animal influenza virus circulating in domesticated or wild animals has caused infection in humans. The WHO consider this a potential pandemic threat.
  • Phase 3: An animal or human-animal influenza virus has caused disease in small clusters of people. However, it has not resulted in human-to-human transmission that is rapid enough to sustain community-level outbreaks.
  • Phase 4: The WHO verify that human-to-human transmission of an animal or human-animal influenza virus is now able to sustain community-level outbreaks.
  • Phase 5: The same virus has caused sustained community-level outbreaks in two or more countries within a single WHO region.
  • Phase 6: In addition to the phase 5 criteria, the same virus has caused sustained community-level outbreaks in at least one other country in a different WHO region.
    • Post-peak period: Levels of pandemic influenza in most countries with adequate surveillance have dropped below peak levels.
    • Post-pandemic period: Levels of influenza activity have returned to the usual levels of seasonal influenza in most countries with adequate surveillance.

According to these definitions, the COVID-19 pandemic is currently in phase 6.


Medical science has advanced rapidly in recent years, but it is unlikely ever to offer full protection from a possible pandemic because of the novel nature of the diseases involved.

Humans would not have natural immunity to a newly mutated disease, meaning that it could have severe effects after spreading between people.

The following have been or continue to be potential causes of concern:


Coronaviruses have caused concern due to their potential to lead to pandemics in recent years. Examples of coronavirus infections, in addition to SARS-CoV-2, include SARS and MERS. In March 2020, SARS-CoV-2 was the first coronavirus to reach a pandemic level, by causing COVID-19.

Previously, health agencies and government bodies managed to prevent coronavirus infections from becoming more than localized epidemics. MERS is still active, but outbreaks occur on a much smaller scaleTrusted Source and less frequently.

COVID-19, on the other hand, has reached every continent except Antarctica.

Viral hemorrhagic fevers

Viral hemorrhagic fevers, including those that the Ebola and Marburg viruses cause, could become pandemics. However, close contact is necessary for these diseases to spread.

Modern surveillance systems, lessons from the most recent Ebola outbreak in West Africa, and an experimental vaccine offer hope that authorities can deal with future outbreaks swiftly, increasing the chances of disease containment.

Antibiotic resistance

Antibiotic resistance is also a major concern. Resistant strains of tuberculosis are among the most worrying.

A 2016 study estimated that almost half a millionTrusted Source new cases of multidrug-resistant tuberculosis (MDR-TB) occurred globally in 2013.


Wild birds are natural hosts for a variety of influenza strains.

Rarely, these influenza strains pass from birds to humans, sparking epidemics with the potential to become pandemics without active surveillance and containment measures.

Avian flu (H5N1) is an example of this. Authorities first identified the strain in Vietnam in 2004Trusted Source. It never progressed beyond epidemic levels, but the virus’s potential ability to combine with human flu viruses is a concern to scientists.


The largest Ebola epidemicTrusted Source took place in Liberia and the surrounding West African nations from 2014 to 2015.

Significant efforts to contain the spread prevented Ebola from turning into a pandemic, even though some people developed the infection overseas.

Ebola has recently resurfaced in the Democratic Republic of Congo, and the WHO are monitoring the situationTrusted Source.


Pandemics are diseases that break out on a global scale. Diseases that spread from animals to humans are often the cause.

From Europe’s Black Death during the Middle Ages to the Spanish flu around the time of the First World War, pandemics can change the course of society for many years to come.

The current pandemic, COVID-19, is causing disruption across the entire world.


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