A pandemic is an outbreak of global proportions. It happens when a bacterium or novel virus becomes capable of spreading rapidly.

It causes serious illness and can spread easily from one person to the next.

The word pandemic comes from the Greek pandemos meaning "pertaining to all people." The Greek word pan means "all" and the Greek word demos means "people."

This article discusses the difference between epidemics and pandemics, how pandemics start, and future concerns.

Fast facts on pandemics

Here are some key points about pandemics.

  • Pandemics are usually caused by a novel infectious agent, an infectious agent that is newly capable of spreading rapidly, or both.
  • The death toll in a pandemic is generally higher than that in an epidemic.
  • The Spanish flu was the worst pandemic in history, killing 100 million people.
  • Increased travel and mobility have increased the likelihood of new diseases spreading.
  • Antibiotic resistance increases the risk of future pandemics.

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The death toll of a pandemic is usually much greater than that of an epidemic.

A pandemic is when a disease spreads across a wide geographical area and affects many people.

An epidemic is specific to one city, region, or country, but a pandemic spreads beyond national borders, possibly worldwide.

An endemic disease is one that is always present in a particular place or community.

An epidemic is when the number of people who experience an infection is higher than the number expected within a country or a part of a country.

If an infection becomes widespread in several countries at the same time, it can become a pandemic.

A pandemic is usually caused by a new virus strain or subtype that becomes easily transmissable between humans, or by bacteria that become resistant to antibiotic treatment. Sometimes, pandemics are caused simply by a new ability to spread rapidly, such as with the Black Death.

Humans may have little or no immunity against a new virus. Often a new virus cannot spread between people, but if it changes, or mutates, it may start to spread easily. In this case, a pandemic can result.

In the case of influenza, seasonal outbreaks — or epidemics — are generally caused by subtypes of a virus that is already circulating among people.

Pandemics, on the other hand, are generally caused by novel subtypes. These subtypes have not circulated among people before.

A pandemic affects more people and can be more deadly than an epidemic. It can also lead to more social disruption, economic loss, and general hardship.

One type of pandemic is that which that can emerge when a type of influenza virus, known as the influenza A virus, changes suddenly, resulting in a virus that is different from any virus that already exists. This is called an antigenic shift.

On the surface of the virus are HA proteins and NA proteins. If one or both of these change, a new influenza A virus subtype can result.

If this subtype gains the ability to spread between people, a pandemic can result.

After the pandemic emerges and spreads, humans develop some immunity. Then, the virus subtype can circulate among humans for several years, causing occasional flu epidemics.

Various bodies around the world, such as the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) monitor the behavior and movements of the virus.

The Spanish flu pandemic, from 1918 to 1920, claimed 100 million lives. It is considered the worst in history. The Black Death claimed the lives of over 75 million people in the 14th Century.

Some pandemics and epidemics that have occurred include:

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The Black Death killed 30-60% of Europe's total population.

  • Plague of Justinian 541
  • Black Death 1346-1350
  • Cholera 1899-1923
  • Spanish flu (H1N1) 1918-1920
  • Asian flu (H2N2) 1957-1958
  • Hong Kong flu 1968-1969
  • Avian flu (H1N1) 2009

Some viruses are present in animals but rarely spread to humans. Sometimes an event can happen that makes this possible.

Health authorities are concerned when a case arises of an animal virus passing to humans, as this can be an indication that the virus is changing.

Swine flu and bird — or avian — flu, refer to viruses that were common in pigs or birds, but not in humans, until an antigenic shift occurred.

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

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The etiology of pandemics can be split into six stages.

The WHO has a six-stage influenza program:

  • Stage 1
    No animal influenza virus circulating among animals has been reported to cause infection in humans.
  • Stage 2
    An animal influenza virus circulating in domesticated or wild animals is known to have caused infection in humans and is therefore considered a specific potential pandemic threat.
  • Stage 3
    An animal or human-animal influenza reassortant virus has caused sporadic cases or small clusters of disease in people, but it has not resulted in human-to-human transmission sufficient to sustain community-level outbreaks.
  • Stage 4
    Human-to-human transmission of an animal or human-animal influenza reassortant virus able to sustain community-level outbreaks has been verified.
  • Stage 5
    The same identified virus has caused sustained community level outbreaks in two or more countries in one WHO region.
  • Phase 6
    In addition to the criteria defined in Phase 5, the same virus has caused sustained community level outbreaks in at least one other country in another 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 levels seen for seasonal influenza in most countries with adequate surveillance.

If an influenza pandemic were to emerge today, the following problems could arise:

  • People today are more international mobile and more likely to live in cities than in the past, factors which increase the risk of a virus spreading.
  • Faster communication increases the risk of panic, and the chance that people who may be infected will travel in an attempt to escape the disease, potentially taking the virus with them.
  • It can take months or years for a vaccine to become available, because pandemic viruses are novel agents.
  • Medical facilities would be overwhelmed, and there could be shortages of personnel to provide vital community services, due to both the demand and illness.

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Pandemics involve novel agents; because of this, medical science must keep on its toes.

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.

The following are all potential causes of concern:

Viral hemorrhagic fevers

Viral hemorrhagic fevers, including the Ebola and Marburg viruses, could become pandemics.

However, close contact is needed for these diseases to spread.

Modern surveillance systems, lessons learned from the Ebola outbreak in West Africa in 2014 to 2015, and an experimental vaccine that is currently available for people who may be affected by the disease, offer hope that, in future, new cases will be dealt with swiftly and that the disease can be contained.

Antibiotic resistance

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

Each year, almost half a million new cases of multidrug-resistant tuberculosis (MDR-TB) are estimated to occur globally.


SARS, caused by the coronavirus, has come close to generating a pandemic in recent years. Health agencies and government bodies prevented the disease from becoming more than localized epidemics. SARS has not been eradicated, however, and may return.

Another respiratory disease, MERS, is also a matter of concern, although the number of cases so far has been small.


Wild birds are a natural host for a variety of influenza strains. In rare cases, these influenza species can pass from bird to human, sparking epidemics with the potential to turn into pandemics, if left unchecked.

Avian flu (H5N1) is an example of this. The strain was first identified in Vietnam in 2004. It never reached epidemic levels, but the potential ability of the virus to combine with human flu viruses is a concern to scientists.


The largest Ebola epidemic the world has ever seen affected Liberia and surrounding countries in West Africa in 2014 to 2015. Huge efforts to contain the problem prevented it from turning into a pandemic.

Ebola has recently resurfaced in the Democratic Republic of Congo, in Central Africa, and the WHO is monitoring the situation.