A pandemic (from Greek πᾶν pan "all" and δῆμος demos "people") is an epidemic of infectious disease that has spread through human populations across a large region; for instance multiple continents, or even worldwide. A widespread endemic disease that is stable in terms of how many people are getting sick from it is not a pandemic. Further, flu pandemics generally exclude recurrences of seasonal flu. Throughout history there have been a number of pandemics, such as smallpox and tuberculosis. More recent pandemics include the HIV pandemic as well as the 1918 and 2009H1N1 pandemics.
A pandemic is an epidemic occurring on a scale which crosses international boundaries, usually affecting a large number of people.
The World Health Organization (WHO) has a six-stage classification that describes the process by which a novel influenza virus moves from the first few infections in humans through to a pandemic. This starts with the virus mostly infecting animals, with a few cases where animals infect people, then moves through the stage where the virus begins to spread directly between people, and ends with a pandemic when infections from the new virus have spread worldwide.
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 infectious or contagious.
In a virtual press conference in May 2009 on the influenza pandemic, Dr 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 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, revised in 2005 and in February 2009, defining phases and appropriate actions for each phase in an aide memoir entitled WHO pandemic phase descriptions and main actions by phase. The 2009 revision, including definitions of a pandemic and the phases leading to its declaration, were finalized in February 2009. The pandemic H1N1 2009 virus, 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.
HIV originated in Africa, and spread to the United States via Haiti between 1966 and 1972.AIDS is currently a pandemic, with infection rates as high as 25% in southern and eastern Africa. In 2006 the HIV prevalence rate among pregnant women in South Africa was 29.1%. Effective education about safer sexual practices and bloodborne infection precautions training have helped to slow down infection rates in several African countries sponsoring national education programs. Infection rates are rising again in Asia and the Americas. AIDS death toll in Africa may reach 90–100 million by 2025.
Pandemics and notable epidemics through history
There have been a number of significant pandemics recorded in human history, generally zoonoses which came about with domestication of animals, such as influenza and tuberculosis. There have been a number of particularly significant epidemics that deserve mention above the "mere" destruction of cities:
Plague of Athens, 430 BC. Possibly typhoid fever killed a quarter of the Athenian troops, and a quarter of the population over four years. This disease fatally weakened the dominance of Athens, but the sheer virulence of the disease prevented its wider spread; i.e. it killed off its hosts at a rate faster than they could spread it. The exact cause of the plague was unknown for many years. In January 2006, researchers from the University of Athens analyzed teeth recovered from a mass grave underneath the city, and confirmed the presence of bacteria responsible for typhoid.
Bubonic plague victims in a mass grave from 1720–1721 in Martigues, France
Antonine Plague, 165–180. Possibly smallpox brought to the Italian peninsula by soldiers returning from the Near East; it killed a quarter of those infected, and up to five million in all. At the height of a second outbreak, the Plague of Cyprian (251–266), which may have been the same disease, 5,000 people a day were said to be dying in Rome.
Black Death, started 14th century. The total number of deaths worldwide is estimated at 75 million people. Eight hundred years after the last outbreak, the plague returned to Europe. Starting in Asia, the disease reached Mediterranean and western Europe in 1348 (possibly from Italian merchants fleeing fighting in Crimea), and killed an estimated 20 to 30 million Europeans in six years; a third of the total population, and up to a half in the worst-affected urban areas. It was the first of a cycle of European plague epidemics that continued until the 18th century. There were more than 100 plague epidemics in Europe in this period. The disease recurred in England every two to five years from 1361 to 1480. By the 1370s, England's population was reduced by 50%. The Great Plague of London of 1665–66 was the last major outbreak of the plague in England. The disease killed approximately 100,000 people, 20% of London's population.
Encounters between European explorers and populations in the rest of the world often introduced local epidemics of extraordinary virulence. Disease killed part of the native population of the Canary Islands in the 16th century (Guanches). Half the native population of Hispaniola in 1518 was killed by smallpox. Smallpox also ravaged Mexico in the 1520s, killing 150,000 in Tenochtitlán alone, including the emperor, and Peru in the 1530s, aiding the European conquerors.Measles killed a further two million Mexican natives in the 17th century. In 1618–1619, smallpox wiped out 90% of the Massachusetts Bay Native Americans. During the 1770s, smallpox killed at least 30% of the Pacific Northwest Native Americans. Smallpox epidemics in 1780–1782 and 1837–1838 brought devastation and drastic depopulation among the Plains Indians. Some believe that the death of up to 95% of the Native American population of the New World was caused by Old World diseases such as smallpox, measles, and influenza. Over the centuries, the Europeans had developed high degrees of immunity to these diseases, while the indigenous peoples had no such immunity.
Researchers concluded that syphilis was carried from the New World to Europe after Columbus' voyages. The findings suggested Europeans could have carried the nonvenereal tropical bacteria home, where the organisms may have mutated into a more deadly form in the different conditions of Europe. The disease was more frequently fatal than it is today. Syphilis was a major killer in Europe during the Renaissance. Between 1602 and 1796, the Dutch East India Company sent almost a million Europeans to work in Asia. Ultimately, only less than one-third made their way back to Europe. The majority died of diseases. Disease killed more British soldiers in India than war.
Since it became widespread in the 19th century, cholera has killed tens of millions of people.
First cholera pandemic 1816–1826. Previously restricted to the Indian subcontinent, the pandemic began in Bengal, then spread across India by 1820. 10,000 British troops and countless Indians died during this pandemic. It extended as far as China, Indonesia (where more than 100,000 people succumbed on the island of Java alone) and the Caspian Sea before receding. Deaths in India between 1817 and 1860 are estimated to have exceeded 15 million persons. Another 23 million died between 1865 and 1917. Russian deaths during a similar period exceeded 2 million.
In 1892, cholera contaminated the water supply of Hamburg, and caused 8,606 deaths.
Sixth pandemic 1899–1923. Had little effect in Europe because of advances in public health, but Russia was badly affected again (more than 500,000 people dying of cholera during the first quarter of the 20th century). The sixth pandemic killed more than 800,000 in India. The 1902–1904 cholera epidemic claimed over 200,000 lives in the Philippines.
The Greek physician Hippocrates, the "Father of Medicine", first described influenza in 412 BC.
The first influenza pandemic was recorded in 1580 and since then influenza pandemics occurred every 10 to 30 years.
The 1889–1890 flu pandemic, also known as Russian Flu, was first reported in May 1889 in Bukhara, Uzbekistan. By October, it had reached Tomsk and the Caucasus. It rapidly spread west and hit North America in December 1889, South America in February–April 1890, India in February–March 1890, and Australia in March–April 1890. The H3N8 and H2N2 subtypes of the Influenza A virus have each been identified as possible causes. It had a very high attack and mortality rate, causing around a million fatalities.
The "Spanish flu", 1918–1919. First identified early in March 1918 in US troops training at Camp Funston, Kansas. By October 1918, it had spread to become a worldwide pandemic on all continents, and eventually infected about one-third of the world's population (or ≈500 million persons). Unusually deadly and virulent, it ended nearly as quickly as it began, vanishing completely within 18 months. In six months, some 50 million were dead; some estimates put the total of those killed worldwide at over twice that number. About 17 million died in India, 675,000 in the United States and 200,000 in the UK. The virus was recently reconstructed by scientists at the CDC studying remains preserved by the Alaskan permafrost. The H1N1 virus has a small, but crucial structure that is similar to the Spanish Flu.
The "Asian Flu", 1957–58. An H2N2 virus caused about 70,000 deaths in the United States. First identified in China in late February 1957, the Asian flu spread to the United States by June 1957. It caused about 2 million deaths globally.
The "Hong Kong Flu", 1968–69. An H3N2 caused about 34,000 deaths in the United States. This virus was first detected in Hong Kong in early 1968, and spread to the United States later that year. This pandemic of 1968 and 1969 killed approximately one million people worldwide. Influenza A (H3N2) viruses still circulate today.
Typhus is sometimes called "camp fever" because of its pattern of flaring up in times of strife. (It is also known as "gaol fever" and "ship fever", for its habits of spreading wildly in cramped quarters, such as jails and ships.) Emerging during the Crusades, it had its first impact in Europe in 1489, in Spain. During fighting between the Christian Spaniards and the Muslims in Granada, the Spanish lost 3,000 to war casualties, and 20,000 to typhus. In 1528, the French lost 18,000 troops in Italy, and lost supremacy in Italy to the Spanish. In 1542, 30,000 soldiers died of typhus while fighting the Ottomans in the Balkans.
During the Thirty Years' War (1618–1648), about 8 million Germans were killed by bubonic plague and typhus. The disease also played a major role in the destruction of Napoleon's Grande Armée in Russia in 1812. During the retreat from Moscow, more French military personnel died of typhus than were killed by the Russians. Of the 450,000 soldiers who crossed the Neman on 25 June 1812, less than 40,000 returned. More military personnel were killed from 1500–1914 by typhus than from military action. In early 1813 Napoleon raised a new army of 500,000 to replace his Russian losses. In the campaign of that year, over 219,000 of Napoleon's soldiers died of typhus. Typhus played a major factor in the Irish Potato Famine. During World War I, typhus epidemics killed over 150,000 in Serbia. There were about 25 million infections and 3 million deaths from epidemic typhus in Russia from 1918 to 1922. Typhus also killed numerous prisoners in the Nazi concentration camps and Soviet prisoner of war camps during World War II. More than 3.5 million Soviet POWs died in the Nazi custody out of 5.7 million.
Smallpox is a highly contagious disease caused by the Variola virus. The disease killed an estimated 400,000 Europeans per year during the closing years of the 18th century. During the 20th century, it is estimated that smallpox was responsible for 300–500 million deaths. As recently as early 1950s an estimated 50 million cases of smallpox occurred in the world each year. After successful vaccination campaigns throughout the 19th and 20th centuries, the WHO certified the eradication of smallpox in December 1979. To this day, smallpox is the only human infectious disease to have been completely eradicated, and one of two infectious viruses ever to be eradicated.
Historically, measles was prevalent throughout the world, as it is highly contagious. According to the National Immunization Program, 90% of people were infected with measles by age 15. Before the vaccine was introduced in 1963, there were an estimated 3–4 million cases in the U.S. each year. Measles killed around 200 million people worldwide over the last 150 years. In 2000 alone, measles killed some 777,000 worldwide out of 40 million cases globally.
Measles is an endemic disease, meaning that it has been continually present in a community, and many people develop resistance. In populations that have not been exposed to measles, exposure to a new disease can be devastating. In 1529, a measles outbreak in Cuba killed two-thirds of the natives who had previously survived smallpox. The disease had ravaged Mexico, Central America, and the Inca civilization.
One–third of the world's current population has been infected with Mycobacterium tuberculosis, and new infections occur at a rate of one per second. About 5–10% of these latent infections will eventually progress to active disease, which, if left untreated, kills more than half of its victims. Annually, 8 million people become ill with tuberculosis, and 2 million people die from the disease worldwide. In the 19th century, tuberculosis killed an estimated one-quarter of the adult population of Europe; by 1918 one in six deaths in France were still caused by TB. During the 20th century, tuberculosis killed approximately 100 million people. TB is still one of the most important health problems in the developing world.
Historically, leprosy has affected people since at least 600 BC. Leprosy outbreaks began to occur in Western Europe around 1000 CE. Numerous leprosaria, or leper hospitals, sprang up in the Middle Ages; Matthew Paris estimated that in the early 13th century there were 19,000 across Europe.
Malaria is widespread in tropical and subtropical regions, including parts of the Americas, Asia, and Africa. Each year, there are approximately 350–500 million cases of malaria.Drug resistance poses a growing problem in the treatment of malaria in the 21st century, since resistance is now common against all classes of antimalarial drugs, except for the artemisinins.
Malaria was once common in most of Europe and North America, where it is now for all purposes non-existent. Malaria may have contributed to the decline of the Roman Empire. The disease became known as "Roman fever".Plasmodium falciparum became a real threat to colonists and indigenous people alike when it was introduced into the Americas along with the slave trade. Malaria devastated the Jamestown colony and regularly ravaged the South and Midwest. By 1830 it had reached the Pacific Northwest. During the American Civil War, there were over 1.2 million cases of malaria among soldiers of both sides. The southern U.S. continued to be afflicted with millions of cases of malaria into the 1930s.
Yellow fever has been a source of several devastating epidemics. Cities as far north as New York, Philadelphia, and Boston were hit with epidemics. In 1793, one of the largest yellow fever epidemics in U.S. history killed as many as 5,000 people in Philadelphia—roughly 10% of the population. About half of the residents had fled the city, including President George Washington. In colonial times, West Africa became known as "the white man's grave" because of malaria and yellow fever.
There are also a number of unknown diseases that were extremely serious but have now vanished, so the etiology of these diseases cannot be established. The cause of English Sweat in 16th-century England, which struck people down in an instant and was more greatly feared than even the bubonic plague, is still unknown.
Viruses causing viral hemorrhagic fever such as Lassa fever virus, Rift Valley fever, Marburg virus, Ebola virus and Bolivian hemorrhagic fever are highly contagious and deadly diseases, with the theoretical potential to become pandemics. Their ability to spread efficiently enough to cause a pandemic is limited, however, as transmission of these viruses requires close contact with the infected vector, and the vector only has a short time before death or serious illness. Furthermore, the short time between a vector becoming infectious and the onset of symptoms allows medical professionals to quickly quarantine vectors, and prevent them from carrying the pathogen elsewhere. Genetic mutations could occur, which could elevate their potential for causing widespread harm; thus close observation by contagious disease specialists is merited.
Antibiotic-resistant microorganisms, sometimes referred to as "superbugs", may contribute to the re-emergence of diseases which are currently well controlled. For example, cases of tuberculosis that are resistant to traditionally effective treatments remain a cause of great concern to health professionals. Every year, nearly half a million new cases of multidrug-resistant tuberculosis (MDR-TB) are estimated to occur worldwide. China and India have the highest rate of multidrug-resistant TB. The World Health Organization (WHO) reports that approximately 50 million people worldwide are infected with MDR TB, with 79 percent of those cases resistant to three or more antibiotics. In 2005, 124 cases of MDR TB were reported in the United States. Extensively drug-resistant tuberculosis (XDR TB) was identified in Africa in 2006, and subsequently discovered to exist in 49 countries, including the United States. There are about 40,000 new cases of XDR-TB per year, the WHO estimates.
In the past 20 years, common bacteria including Staphylococcus aureus, Serratia marcescens and Enterococcus, have developed resistance to various antibiotics such as vancomycin, as well as whole classes of antibiotics, such as the aminoglycosides and cephalosporins. Antibiotic-resistant organisms have become an important cause of healthcare-associated (nosocomial) infections (HAI). In addition, infections caused by community-acquired strains of methicillin-resistant Staphylococcus aureus (MRSA) in otherwise healthy individuals have become more frequent in recent years.
In 2003, there were concerns that Severe Acute Respiratory Syndrome (SARS), a new and highly contagious form of atypical pneumonia, might become pandemic. It is caused by a coronavirus dubbed SARS-CoV. Rapid action by national and international health authorities such as the World Health Organization helped to slow transmission and eventually broke the chain of transmission, which ended the localized epidemics before they could become a pandemic. However, the disease has not been eradicated. It could re-emerge. This warrants monitoring and reporting of suspicious cases of atypical pneumonia.
Wild aquatic birds are the natural hosts for a range of influenza A viruses. Occasionally, viruses are transmitted from these species to other species, and may then cause outbreaks in domestic poultry or, rarely, in humans.
In February 2004, avian influenza virus was detected in birds in Vietnam, increasing fears of the emergence of new variant strains. It is feared that if the avian influenza virus combines with a human influenza virus (in a bird or a human), the new subtype created could be both highly contagious and highly lethal in humans. Such a subtype could cause a global influenza pandemic, similar to the Spanish Flu, or the lower mortality pandemics such as the Asian Flu and the Hong Kong Flu.
From October 2004 to February 2005, some 3,700 test kits of the 1957 Asian Flu virus were accidentally spread around the world from a lab in the US.
In May 2005, scientists urgently called upon nations to prepare for a global influenza pandemic that could strike as much as 20% of the world's population.
In October 2005, cases of the avian flu (the deadly strain H5N1) were identified in Turkey. EU Health Commissioner Markos Kyprianou said: "We have received now confirmation that the virus found in Turkey is an avian flu H5N1 virus. There is a direct relationship with viruses found in Russia, Mongolia and China." Cases of bird flu were also identified shortly thereafter in Romania, and then Greece. Possible cases of the virus have also been found in Croatia, Bulgaria and the United Kingdom.
By November 2007, numerous confirmed cases of the H5N1 strain had been identified across Europe. However, by the end of October only 59 people had died as a result of H5N1 which was atypical of previous influenza pandemics.
Avian flu cannot yet be categorized as a "pandemic", because the virus cannot yet cause sustained and efficient human-to-human transmission. Cases so far are recognized to have been transmitted from bird to human, but as of December 2006 there have been very few (if any) cases of proven human-to-human transmission. Regular influenza viruses establish infection by attaching to receptors in the throat and lungs, but the avian influenza virus can only attach to receptors located deep in the lungs of humans, requiring close, prolonged contact with infected patients, and thus limiting person-to-person transmission.
In 1346, the bodies of Mongol warriors who had died of plague were thrown over the walls of the besieged Crimean city of Kaffa (now Theodosia). After a protracted siege, during which the Mongol army under Jani Beg was suffering the disease, they catapulted the infected corpses over the city walls to infect the inhabitants. It has been speculated that this operation may have been responsible for the arrival of the Black Death in Europe.
During the Sino-Japanese War (1937–1945), Unit 731 of the Imperial Japanese Army conducted human experimentation on thousands, mostly Chinese. In military campaigns, the Japanese army used biological weapons on Chinese soldiers and civilians. Plague fleas, infected clothing, and infected supplies encased in bombs were dropped on various targets. The resulting cholera, anthrax, and plague were estimated to have killed around 400,000 Chinese civilians.
Spores of weaponized anthrax were accidentally released from a military facility near the Soviet closed city of Sverdlovsk in 1979. The Sverdlovsk anthrax leak is sometimes called "biological Chernobyl". In January 2009, an Al-Qaeda training camp in Algeria was reportedly wiped out by the plague, killing approximately 40 Islamic extremists. Some experts said that the group was developing biological weapons, however, a couple of days later the Algerian Health Ministry flatly denied this rumour stating "No case of plague of any type has been recorded in any region of Algeria since 2003".
^Stéphane Barry and Norbert Gualde, in L'Histoire n° 310, June 2006, pp.45–46, say "between one-third and two-thirds"; Robert Gottfried (1983). "Black Death" in Dictionary of the Middle Ages, volume 2, pp.257–67, says "between 25 and 45 percent".
^Stein CE, Birmingham M, Kurian M, Duclos P, Strebel P (May 2003). "The global burden of measles in the year 2000—a model that uses country-specific indicators". J. Infect. Dis.187 (Suppl 1): S8–14. doi:10.1086/368114. PMID12721886.
^Man and Microbes: Disease and Plagues in History and Modern Times; by Arno Karlen