A pandemic (from Greek παν pan all + δήμος demos people) is an epidemic of infectious disease that spreads through human populations across a large region; for instance a continent, or even worldwide.

Definition

According to the World Health Organization (WHO), a pandemic can start when three conditions have been met:

• the emergence of a disease new to the population.
• the agents infects humans, causing serious illness.
• the agents spreads easily and sustainably among humans.

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 (although certain cases of some types of cancer might be).

WHO pandemic influenza phases

The World Health Organization Global Influenza Preparedness Plan defines the stages of pandemic influenza, outlines the role of the WHO and makes recommendations for national measures before and during a pandemic. The phases are:

Interpandemic period:

• Phase 1: No new influenza virus subtypes have been detected in humans.
• Phase 2: No new influenza virus subtypes have been detected in humans, but an animal variant threatens human disease.

Pandemic alert period:

• Phase 3: Human infection(s) with a new subtype, but no human-to-human spread.
• Phase 4: Small cluster(s) with limited localized human-to-human transmission.
• Phase 5: Larger cluster(s), but human-to-human spread still localized.

Pandemic period:

• Phase 6: Increased and sustained transmission to general population.

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:

• Peloponnesian War, 430 BC. 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. 1
• 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 (251–266), 5,000 people a day were said to be dying in Rome.
• Plague of Justinian, from 541 to 750, was the first recorded outbreak of the bubonic plague. It started in Egypt, and reached Constantinople the following spring, killing (according to the Byzantine chronicler Procopius) 10,000 a day at its height, and perhaps 40% of the city's inhabitants. The plague went on to eliminate a quarter to a half of the human population that it struck throughout the known world. It caused Europe's population to drop by around 50% between 550 and 700.
• Black Death, started 1300s. Eight hundred years after the last outbreak, the bubonic plague returned to Europe. Starting in Asia, the disease reached Mediterranean and western Europe in 1348 (possibly from Italian merchants fleeing fighting in the Crimea), and killed 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.
• Cholera
• The First Pandemic 1816 – 1826. Previously restricted to the Indian subcontinent, the pandemic began in Bengal, then spread across India by 1820. It extended as far as China and the Caspian Sea before receding.
• The Second Pandemic (1829–1851) reached Europe (London) in 1832, Canada (Ontario), and USA (New York) in the same year, and the Pacific coast of North America by 1834.
• The Third Pandemic (1852–1860) mainly affected Russia, with over a million deaths.
• The Fourth Pandemic (1863–1875) spread mostly in Europe and Africa.
• In 1866, there was an outbreak in North America.
• In 1892, cholera contaminated the water supply of Hamburg, Germany, and caused 8,606 deaths.
• The seventh pandemic (1899–1923) had little effect in Europe because of advances in public health, but Russia was badly affected again.
• The eighth pandemic began in Indonesia in 1961, called El Tor after the strain, and reached Bangladesh in 1963, India in 1964, and the USSR in 1966.
• Influenza
• The "first" pandemic of 1510 travelled from Africa and spread across Europe.
• The "Asiatic Flu", 1889–1890. Was first reported in May of 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. It was purportedly caused by the H2N8 type of flu virus, and had a very high attack and mortality rate.
• 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 world-wide pandemic on all continents, and eventually infected 2.5 to 5% of the human population, with 20% or more of the world population suffering from the disease to some extent. Unusually deadly and virulent, it ended nearly as quickly as it began, vanishing completely within 18 months. In six months, 25 million were dead; some estimates put the total of those killed worldwide at over twice that number. An estimated 17 million died in India, 500,000 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. They identified it as a type of H1N1 virus.Fact: date=October 2007
• The "Asian Flu", 1957–58. An H2N2 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.
• 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. Influenza A (H3N2) viruses still circulate today.
• Typhus, 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 people died of typhus while fighting the Ottomans in the Balkans. The disease also played a major role in the destruction of Napoleon's Grande Armée in Russia in 1812. Typhus also killed numerous prisoners in the Nazi concentration camps and Soviet prisoner of war camps during World War II.
• Effects of Colonization. Encounters between European explorers and populations in the rest of the world often introduced local epidemics of extraordinary virulence. Disease killed the entire native (Guanches) population of the Canary Islands in the 16th century. 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 1600s. Some believe that the death of 90 to 95 percent of the Native American population of the New World was caused by Old World diseases. As late as 1848–49, as many as 40,000 out of 150,000 Hawaiians are estimated to have died of measles, whooping cough and influenza.
• Dengue. Spread of Dengue disease in South Asia by a mosquito.